Past limit electric power generation costs and past limit electric assist costs are stored as pieces of planning information, based on each of which a fuel consumption amount is estimated assuming that a vehicle travels according to a past traveling pattern. An average estimated fuel consumption amount is computed by averaging the fuel consumption amounts estimated for the pieces of planning information. The piece of planning information including the lowest average estimated fuel consumption amount is selected for the next traveling. An improvement amount during electric power generation and an improvement amount during assistance are computed based on the selected piece of planning information (limit electric power generation cost), the limit electric assist cost, and the present generating and electric assist costs required actually while the vehicle is traveling. An engine and a motor/generator of the vehicle are controlled based on the two improvement amounts.