The incident occurred on Saturday, when Kishida was about to give a speech at a fishing port in the western city of Wakayama. The suspect, Kimura Ryuji, was arrested on the spot.
Police say the device may have been a pipe bomb made of metal. They believe Kimura built it himself, as they found tools and what appears to be gunpowder and metal pipes at his home during a search.
Police are analyzing Kimura's computer and cell phone to determine how he obtained the materials and how he made the device.
Tackling harmful online content
The incident comes after former Prime Minister Abe Shinzo was shot dead at a political rally in July 2022.
A man indicted over the shooting is believed to have found information online to make the gun used in the crime.
The National Police Agency has since stepped up its online monitoring, including a center that looks into and works to remove illegal or harmful material.
Content related to gun and bomb-making was added in March to the list of harmful information the center monitors. Posts that appear to be soliciting others to commit murder, robbery or human trafficking were also included.
Bystanders helped restrain suspect
Several people who were at the campaign event helped restrain the suspect.
One 54-year-old man described how he helped detain Kimura at the scene.
He told reporters on Sunday that he held the suspect down with all his strength when he continued to move his hands after throwing something.
"The suspect didn't speak or resist," he said. "He didn't release a cylindrical object he was holding, so I had to knock it away."
He also said he received a phone call from Prime Minister Kishida on Saturday evening, thanking him for his actions.
Family say suspect rarely left home
NHK has learned that the suspect's family has told police he has been mostly staying at home for the past few years.
A source close to the investigation told NHK Kimura's family said in an interview with police that they have no idea why he would carry out such an act.
Multiple residents in the neighborhood say the Kimuras moved in about 15 years ago.
A woman living near the suspect's home said Kimura was polite and greeted her when they ran into each other. She also said she often saw him gardening with his mother.
One of the suspect's classmates at junior high school told NHK Kimura always seemed to be alone.
Another former classmate says she doesn't remember speaking with him much, and that he was quiet at school.
Suspect shared his hopes in school essay
Some details are emerging about Kimura's past. About to finish elementary school, he wrote a short essay saying his dream was to become either a "pastry chef or an inventor."
He wrote that he would like to make confectionery so delicious people would want it all to themselves.
He also expressed his wish to develop a safer car, and robots to support older people living by themselves.
Later, when finishing junior high school in March 2014, Kimura wrote on the subject, "To myself 10 years from now." He wrote about light, saying, "Humans often associate light with hope, which reflects their deep dependence on light."
He continued, "Black holes are pitch-black because they are celestial bodies from which substances, including light, cannot escape due to the powerful gravity."
He also wrote that time machines could not be created, as no technology would lead to the invention of a device capable of traveling faster than light.
Government says all necessary measures taken
Chief Cabinet Secretary Matsuno Hirokazu said at a news conference on Monday morning that all necessary measures were taken by the National Police Agency and local police in Wakayama Prefecture.
He added that in response to this incident, the agency has instructed police across the country to step up security measures for the G7 Hiroshima summit in May.
Matsuno said all relevant organizations will work together to make sure the G7 summit takes place safely.