A former care home worker accused of mass killings in Japan has rejected his lawyers' argument that he is mentally unfit to stand trial.

The Yokohama District Court began questioning defendant Satoshi Uematsu on Friday. The 30-year-old is charged with killing 19 people with intellectual disabilities and injuring 26 others at the care home in Sagamihara City in July 2016.

The defendant has admitted to the killings. But his lawyers have entered a not guilty plea, on the grounds that he cannot be held legally responsible due to a mental disorder caused by the use of cannabis.

Asked if he understands the plea, Uematsu said it is a mistake to contest his legal competency, and that he is fit to be held responsible.

The defendant said he gave himself up to police to show that he understood he had committed a crime.

Uematsu repeated his claim that he thought it was okay to kill people with severe disabilities without their consent or those of their families.

He said he had confided his murder plot to about 50 acquaintances and drew laughter from more than half of them. He said he took it as a sign of their agreement.

Uematsu also said he thought he could gain what he called "the right to own money" if he killed people with disabilities.

The defendant spoke clearly during the session, wiping away his sweat once in a while. He is due to be questioned again on Monday.