A storing unit stores traveling history information items on a vehicle, the traveling history information items being respectively associated with trips each being a segment from a start of a driving operation to an end of the driving operation. A determination unit determines whether to assign one of the traveling history information items, which is stored in the storing unit and associated with one trip, to a predetermined period, the one trip being partially included in the predetermined period. A driving operation diagnostic unit diagnoses a driving operation of a driver in the predetermined period according to the traveling history information item determined by the determination unit to be assigned to the predetermined period.