Nine dismembered bodies, with as many as 240 bone parts stashed in coolers and toolboxes, has been sprinkled with cat litter in a bid to hide the evidence.
He pleaded guilty to the murders in October, saying the allegations against him were “all correct.” All but one of his victims were women, aged between 15 and 26.
Shiraishi used Twitter, a micro-blogging website, to lure his suicidal victims to his home. He told them he could help them die and claimed he would kill himself alongside them.
Lawyers for Shiraishi said he should only receive a jail sentence because his victims said on social media that they wanted to die, arguing this amounted to consent.
But the court disagreed, ruling that “none of the nine victims consented to be killed, including silent consent.”
“It is extremely grave that the lives of nine young people were taken away. The dignity of the victims was trampled upon,” the judge said.
The father of one 25-year-old victim said in court last month that he “will never forgive Shiraishi even if he dies,” public broadcaster NHK reported at the time.
“Even now, when I see a woman of my daughter’s age, I mistake her for my daughter. This pain will never go away. Give her back to me!” he said.
The case has prompted a huge discussion in Japan on how suicide is discussed and debated online.
Japan is one of few developed countries to retain the death penalty, and public support for it remains high.
Years usually pass between sentencing and execution, which is carried out by hanging in Japan.
The last execution was in December 2019, when a Chinese man was hanged for the murder of a family of four.