PURPOSE: To improve the durability of a pulley and a belt by fixing a primary control valve so that the primary pressure becomes maximum at the time of failure such as disconnection or the like, and preventing the abnormal pressure rise with a relief valve device provided in a circuit between a primary cylinder and the primary control valve.

CONSTITUTION: A non-stage transmission 5 is controlled for speed change on the basis of the primary pressure of a primary control valve 60 to be controlled by a primary control unit 72. In the case that the electrical signal is not input because of disconnection or the like, the primary pressure is made maximum to forcedly fix the non-stage transmission 5 to a high-speed stage of the minimum speed change ratio for fail-safe so that the quick deceleration and a slip of a belt 26 is not generated. At this stage, when the primary pressure rises more than a required value with the secondary pressure, the oil pressure of a primary cylinder 21 is restricted by a relief valve device 81 to avoid an overload of the belt 26 and pulleys 22, 25. Durability of the belt is thereby improved to eliminate unrequired increase of rigidity.