Yasuda shares his experience
On his way back to Japan, he spoke to media on the plane, saying expressing himself in Japanese was difficult as he hadn't spoken the language in 40 months.
He said he believes he was held in Idlib province, a key stronghold of anti-government forces.
"It was hell physically and psychologically, and I gradually lost control of myself with the realization each day that I was still a hostage," he said. "I didn't know how long I would be held, or whether I would come out alive. I just kept thinking negative thoughts, because I was unable to do anything else. I was depressed and always recalling past regrets."
Several videos and images of a man thought to be Yasuda were posted online while he was held captive. The one released in July led to a great deal of speculation. The man in the video said, "My name is Umar. I'm South Korean." Umar is a common name for Muslim men. Yasuda explained.
"There were other captives around me, so if I revealed my name or nationality, the others would hear it. If the other captives were released, they could inform Japanese officials or other organizations about my whereabouts. That's why I was told not to reveal such information."
He continued, "Shooting those videos was like a game. The captors hinted that I might be released if I completed the filming. I identified myself as a South Korean in the video because that's what I was told to do. I chose the name Umar, since I had to become a Muslim during detention due to the circumstances. I just followed the rules the captors had set."
He says, "The captors told me they wanted to shoot 2 versions -- one in which I'm crying and one in which I'm not. But I couldn't cry on command, so I told them to bring chili peppers and rubbed them on my face. Once my nose started running, I was told to say, 'Help me,' so I did."
Who is Yasuda?
Yasuda worked for a newspaper before becoming a freelance writer. He specialized in reporting on conflict zones, including Iraq and Syria.
Yasuda entered Iraq in 2004, when the Self Defense Forces were sent in to help rebuild the country. He was captured by a militant group and released 3 days later.
Yasuda was close to freelance journalist Kenji Goto, who was brutally killed by the Islamic State militants in 2015. After Goto's death was confirmed, he said, "Some people say reporters should not be setting foot in conflict zones, but there are people extending assistance to refugees in conflict areas. People need information to judge whether this is appropriate. Goto's death doesn't in any way negate the importance of reporting on conflict zones."
Four months later, in June 2015, Yasuda went missing after entering Syria from southern Turkey to cover the civil war. It was believed he was being held by a militant group.
Tokyo University of Foreign Studies Professor Hiroyuki Aoyama, an expert on Syria, says the changing situation in the Middle Eastern country may be behind the released of Yasuda.
Aoyama said Syria's government under President Bashar al-Assad has gained the upper hand in the country's civil war, and that an end to the conflict is now in sight.
He said given this situation, holding Yasuda hostage may have become more of a burden for the militants.
He also referred to assistance from the Qatari government in securing Yasuda's release. He said Qatar, like Turkey, supports the opposition forces in the war and can exert influence.
Aoyama said Qatar had played an intermediary role in negotiations with opposition forces on releasing other foreign hostages.
The Syrian Observatory for Human Rights released a comment on Tuesday saying that it had obtained information regarding Yasuda's handover. It says the journalist was handed over to a military faction close to Turkish authorities 4 days ago, but that the announcement of his release was delayed for political purposes.
Rami Abdurrahman, the head of the organization, spoke to NHK. "The government of Qatar informed Japan about Yasuda's release and made it public after Turkey's President Recep Tayyip Erdogan criticized the Saudi Arabian government on October 23rd over the killing of a Saudi journalist."
"I hear that Qatar chose this timing to stress that it had cooperated with Turkey in rescuing Yasuda, while Saudi Arabia, which Qatar is at odds with, allegedly contributed to the death of the journalist."
The question of ransom
Japan's Chief Cabinet Secretary Yoshihide Suga denied that the government directly negotiated with the militants.
"After the incident came to light, the Japanese government asked for cooperation from Qatar, Turkey and other countries, and used various information channels and made an all-out effort. Prime Minister Shinzo Abe made a strong request to the presidents and king for help," he said.
Suga also told reporters that the government didn't pay a ransom.
A Qatari expert on counterterrorism, Jabir Al-Harami, told NHK in Doha that negotiations with the captors took place at the request of the Japanese government and that the talks were held in secret to avoid possible failure.
Harami indicated that the Qatari side may have paid a ransom to the rebel forces. He said he believes ransom payments are no problem if they help save people's lives.
A senior Qatari government official told NHK his country provided information that helped to secure the release of Yasuda. He explained that Qatar and Turkey provided Japan with intelligence through diplomatic routes and counterterrorism agencies.
The official also suggested that his country cooperated with Japan through a contact framework that was launched at the Japanese government's request.
It is not known whether Qatar paid a ransom to the militant group in exchange for Yasuda's release.
Reunited with family
According to his wife Myu, Yasuda ate rice balls and side dishes made by his mother upon arrival at Narita Airport near Tokyo.
He did not speak to the media after arriving, but Myu read out a statement from him.
In the remarks, Yasuda offered an apology for causing tremendous trouble and worrying many people. He thanked people for their help and support in making his return possible.
Yasuda also said that he has a responsibility to give as detailed an account of the incident as possible and that he will do so at the right time.