A hydraulic clutch transmits driving torque in a power train of a vehicle. An oil pressure supply unit supplies oil pressure for engagement to the clutch and a controller controls the unit. The controller causes the unit, prior to engagement of the clutch, to precharge the interior of the clutch with hydraulic fluid. The controller counts an elapsed time since the last release of the clutch and shortens the precharge period as the elapsed time decreases, thereby optimizing the precharge state of the clutch.