Japanese police have identified the detained suspect as Shinji Aoba but he has not yet been arrested.
Media reports say the 41-year-old man seemed angry and shouted "Die!" before he poured a flammable liquid on the entrance to Kyoto Animation headquarters and set it ablaze on Thursday.
The suspected arsonist shouted that he had been plagiarised and had reportedly planned the attack on an animation studio.
The fire was put out nearly a day later, killing 33 people and injuring 10 others in Japan's worst mass killing since 2001.
Japan's Kyodo news agency reported that when the suspect was detained, he confessed to the police and claimed the studio had stolen his novel.

Russian drone, missile attack on Kyiv hits residential buildings
US, Iran talks conclude in Doha, focused on Strait of Hormuz
Trump honours Theodore Roosevelt in North Dakota, debuts new Air Force One
Kenyan court charges 8 schoolgirls with fellow students' murder
At least 5 people killed in fire in Antwerp apartment block