When a gearshift command for a power-on/downshift or power-off/upshift mode is issued, the magnitude of a change in the engine rotational speed due to the gearshift is calculated from the vehicle speed or the engine rotational speed when the gearshift starts to be effected and the gear ratio of a gear position to be reached by the gearshift. If the magnitude of the engine speed change is large, then a hydraulic control pressure is lowered for a certain period of time to advance the timing of operating of a hydraulic pressure release valve to quickly disengage a previous-gear-position clutch. If the magnitude of the engine speed change is small, then the control pressure is increased to delay the timing of operation of the hydraulic pressure release valve to allow the previous-gear-position clutch to be disengaged after a next-gear-position clutch starts to be engaged.