A hydraulic control method is applied to a continuously variable speed change gear mechanism including a pulley having a fixed pulley member fixed to a rotary shaft and a movable pulley member movable by oil pressure toward and away from the fixed pulley member so that a groove width between the fixed and movable pulley members is decreased or increased to thereby reduce or increase the effective rotational radius of a belt extending between the pulley members and thus change a change gear ratio and control the gear change. In this method, first to third three-way electromagnetic pressure control valves control the oil pressure and an electronic control unit (ECU) controls the electromagnetic valves, and driving frequencies of the valves are switched by the ECU in accord with the oil temperature and/or vehicle speed so that the hydraulic clutch is smoothly and effectively operated, the lifetime of the electromagnetic valves is extended, and vibration of the vehicle body due to pressure fluctuation in the hydraulic circuit is prevented.