The bodies of three men aged between 21 and 35, and a 23 year-old woman were found in rented car at an isolated parking lot in Nikko, near a popular shrine late on Monday, the police said.
Hours later, a second car was found in Ninomiya city, about 50 kilometres south-east of Nikko, with the bodies of two men and one woman.
Police found charcoal stoves inside both vehicles, whose windows were sealed with tape.
Inhaling carbon monoxide from charcoal-fueled stoves is a popular way to commit suicide in Japan, where nine people killed themselves in two apparent group suicides the same way in early February.
A government survey indicated that more than 32,000 Japanese committed suicide in 2003, exceeding 30,000 for the sixth straight year.
Most of the suicides were due to bad health or economic problems, such as being heavily indebted. Nearly one-third of the suicides in 2003 were the elderly.