Shiraishi, 29, said she used Twitter and other social media platforms to contact women who had expressed suicidal thoughts.
In 2017, he drew eight women between August and October to his apartment in Zama town, claiming he would help them die, the outlet reported.
Once at his apartment, he sexually assaulted them, strangled them and cut their bodies apart.
The only male victim was the boyfriend of one of the women, who confronted Shiraishi at her apartment to find out where she was, the Guardian reported.
The serial murders were discovered in October 2017 by authorities investigating the disappearance of a young woman, who turned out to be one of the victims.
Officers attended her apartment and found body parts of the victims in coolers and other storage containers, which were covered with kitty litter.
Shiraishi’s admission on Wednesday comes in a series of hearings, which are expected to last 77 days, the Japan Times reported.
If convicted of murder, Shiraishi faces the death penalty for the series of murders, the outlet reported.