A ground surface position lateral acceleration calculator calculates ground surface position lateral acceleration based on lateral acceleration detected by an acceleration sensor, a yaw rate and a roll rate detected by a gyro sensor, yaw angular acceleration and roll angular acceleration calculated by an angular acceleration calculator, a roll angle calculated by a roll angle calculator, and specification information stored in a storage. A vehicle lateral force calculator calculates a vehicle lateral force based on the roll angular acceleration calculated by the angular acceleration calculator, the roll angle calculated by the roll angle calculator, the ground surface position lateral acceleration calculated by the ground surface position lateral acceleration calculator and the specification information stored in the storage.