The 26-year-old defendant, Hattori Kyota, was accused of stabbing a male passenger, then 72, and starting a fire using lighter fluid.
Hattori was charged with attempted murder and arson. The attack took place on a Keio Line train in Chofu City in October 2021.
In a court hearing last month, Hattori said he carried out the attack because he wanted to get the death penalty. He said he had wanted to kill himself after learning his ex-girlfriend got married.
Defense lawyers and prosecutors deliberated on whether the arson attack constituted attempted murder on the 12 passengers in the train carriage.
The prosecution argued the attack was premeditated with the intention to kill. It said Hattori chose a limited express train because it has fewer stops, making it harder for passengers to escape. It demanded 25 years in prison.
The defense said the passengers had already reached safety when Hattori started the fire and were not at risk of death. It said the act did not constitute attempted murder and asked for a 12-year sentence.
The Tokyo District Court's Tachikawa Branch announced the sentence on Monday.
The presiding judge described the attack as an indiscriminate crime carried out for selfish reasons. He said Hattori targeted a running train because there are not many ways to escape and that many more people could have been injured or even killed.
But the court ruled the arson attack did not constitute attempted murder on two of the passengers, who it deemed to have already reached safety.
The judge concluded by addressing Hattori directly: "It's a long prison term. During this time, I want you to think about the incident, the victims, and your own life after you return to society. Whatever hardships you may have, live your life and do not forget to atone."
Halloween night attack
The incident took place on a Keio Line train bound for central Tokyo on Halloween night two years ago.
Hattori was wearing a green shirt, a tie and a purple coat — dressed as the Joker from the popular US comic book series, "Batman."
When police officers arrived on the scene, they found him sitting calmly. He was holding a knife in one hand and smoking a cigarette with the other.
Originally planned to attack people in Shibuya
Over the course of two days in court, Hattori revealed details about his motives and preparation. He said he had initially been planning to attack people in Shibuya, which would be crowded with Halloween revelers.
But he said he changed his mind after hearing of a knife attack on an Odakyu Line train that left 10 people injured. Hattori said he decided to spread gasoline through a train car and light it on fire.
'I wanted the death penalty.'
Hattori said he was motivated by problems with a relationship and his job.
He told the court he had been dating someone he met in junior high school for nine years. He said she was the only person he could trust, citing trauma from bullying.
"We picked out an engagement ring and set a date to sign marriage papers," he said. "But on my birthday, she suddenly said she wanted to end our engagement. Six months later, I found out she had married someone else. I was so shocked and lost the reason for my existence. I wanted to kill myself."
Around the same time, he said he made a mistake at work and was transferred to a different department. He decided to quit.
Hattori said he tried to commit suicide twice, but having failed, he started to think the death penalty was the only way he could die. He said he thought he had to kill people to ensure he got such a sentence.
The prosecution asked him how he felt about doing something that might cause the deaths of other people. Hattori said he felt he had no choice if he wanted the death penalty. He said he was unsure of whether to go ahead with it, and this was why he dressed up as the Joker.
"The character seemed like someone who doesn't care about killing people," he said. "I thought it would be impossible for me to murder someone unless I felt the same way. I decided to become the character."
On the second day of the trial, Hattori expressed remorse for his actions. He said he felt sorry for his victims and regretted how much suffering he had caused.
The prosecution asked why he hadn't apologized until now. He said he thought it would hurt his chances of getting the death penalty.
Victims call for heavy punishment
Victims and other passengers spoke about their experiences that night in court.
The elderly man who Hattori stabbed said his injuries make it hard for him to carry items or write. He said someone who cannot apologize for his actions should bear responsibility.
A man who was 16 years old at the time said he only just managed to escape. "If it was one step later, I might have been killed. I was scared." He called for harsh punishment.
A woman who was 21 years old at the time said she struggled to go outside after the incident.
"I couldn't go to school or do part-time jobs," she said. "I didn't feel like I was alive. Not only me, but my family and the people around me suffered. I don't want to be in the same world as the defendant. I don't want him to ever get out of prison."