PURPOSE: To take a counter measure mechanically through hydraulic control system against the occurrence of a belt slip by providing a shift lock mechanism which moves the port shaft of a shift lock valve when tire lock is executed during brake on a low friction road.

CONSTITUTION: When tire lock is effected during brake on a low friction road, a spool 84b of a shift lock valve 84 is moved through a port shaft 84g through the working of a shift lock mechanism 160 to close a drain port 83d. Even when a primary cylinder is drained by means of a change gear ratio control valve, a primary pressure is held at a high value. In this case, the port shaft 84g is closed and a high primary pressure is exerted on the back of a shift lock valve 84 to self-lock it in a closed position. During release of a brake, a primary pulley and a belt are rotated by means of a press force, generated by a high primary pressure, without the occurrence of a belt slip. After release of a brake, the port shaft 84g is opened by means of the shift lock mechanism 160, and a check valve 83 is released from self-lock to restart drainage of a primary pressure.