A police spokesman said seven young people were found in a van in the Saitama mountains to the west of Tokyo. Two women were found dead in a car south of Tokyo, in another apparent suicide pact.
Japan has recently seen a wave of internet-linked suicides, as people seek companions to die with.
More than 34,000 Japanese took their own lives in 2003, according to the National Police Agency – an increase of more than 7% from the previous year.
Economic difficulties and an increasing sense of isolation among Japanese youth are believed to be contributing to the rise.