Surplus pressure is obtained from a difference between a hydraulic pressure value detected by a hydraulic pressure sensor and a target supplied hydraulic pressure. A determination is made about whether or not a possible hydraulic pressure in a prescribed time will become lower than a minimum required hydraulic pressure based on a current surplus pressure and a changing rate of the surplus pressure. If the possible hydraulic pressure is lower than the minimum required hydraulic pressure, correction is conducted to increase supplied hydraulic pressure. Accordingly, control is conducted so that the regularly required surplus pressure can be reduced to a minimum required amount.