Japanese media called it the "house of horrors" after investigators discovered nine heads along with a large number of arm and leg bones stashed in coolers and toolboxes.
Prosecutors sought the death penalty for Shiraishi, who admitted to killing and butchering his victims.
But Shiraishi's lawyers argued that he was guilty of the lesser charge of "murder with consent", claiming his victims had given their permission to be killed.
Shiraishi later disputed his own defence team's version of events and said he killed without their consent.
On Tuesday, the judge who delivered the verdict said that "none of the victims agreed to be killed".
"The defendant was found to be fully responsible," said Naokuni Yano, reported The Straits Times newspaper.
The father of one victim, aged 25, told the court last month that he "will never forgive Shiraishi even if he dies", according to Japanese broadcaster NHK
"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 had said.
The killings stunned Japan, triggering a fresh debate about websites where suicide is discussed. At the time the government indicated it may introduce new regulations.
The murders also prompted a change by Twitter, which amended its rules to state users should not "promote or encourage suicide or self-harm".
