A control system for a belt-type continuously variable transmission of a motor vehicle, having a primary and a secondary control valve for varying the transmission ratio. The primary control valve receives an electric signal from a control unit to vary a primary hydraulic pressure. The primary control valve produces a primary pressure which causes the transmission to take an intermediate transmission ratio when the electric signal is not transmitted due to a failure such as breakage of a cable. Thus, dangerous situations of the vehicle are prevented, and the vehicle can reliably start in a relatively low speed range of the transmission.