The gear ratio of a belt-type continuously variable transmission is changed by increasing or decreasing the hydraulic pressure applied to an oil chamber for a drive pulley by a ratio control valve. A spool of the ratio control valve is driven by a normally opened solenoid valve and a normally closed solenoid valve. In a lower gear ratio range, the gear ratio is controlled by controlling the duty rate of the normally opened solenoid valve, and in a higher gear ratio range, the gear ratio is controlled by controlling the duty rate of the normally closed solenoid valve. When any trouble is generated, the gear ratio is held at an intermediate value by bringing both the duty rates of the normally opened solenoid valve and the normally closed solenoid valve into 0% or 100%. Thus, it is possible to finely control the gear ratio of the belt-type continuously variable transmission and, when any trouble is generated, the gear ratio can be held at the value intermediate between a LOW value and an OD value.