Takahiro Shiraishi, 29, is also accused of dismembering his victims and storing body parts in cool boxes, did not contest nine counts of murder, saying they “are all correct”, public broadcaster NHK reported.
But lawyers for Takahiro Shiraishi argued the charges should be reduced because the victims – who had expressed suicidal thoughts – wanted him to kill them.
He is accused of using Twitter to contact victims aged between 15 and 26 who talked online about committing suicide, telling them he could help them in their plans – or even die alongside them.
His lawyers want the charges against him reduced to “murder with consent”, which carries a prison sentence of between six months and seven years.
He is also facing rape charges, according to media reports.
Shiraishi was detained three years ago by police investigating the disappearance of a 23-year-old woman who had reportedly tweeted about wanting to take her own life.
After she went missing, her brother apparently gained access to her Twitter account and noticed a suspicious handle.
The profile read: “I want to help people who are really in pain. Please DM me anytime.”
In a post made on October 21, Shiraishi wrote: “Bullying is everywhere, in school and at work.
“There must be many people in society who are suffering after attempting suicides, though their cases are not reported in the news. I want to help such people.”