To provide a control device for a transmission capable of protecting a mechanism of the transmission by quickly detecting the failure of an oil pressure switching means.

In this control device of the transmission comprising a lock-up clutch for controlling the slipping quantity of a fluid transmission device, a specific frictional connecting element engaged in a specific gear shift stage, a single solenoid valve controlling operating states of the lock-up clutch and the specific frictional engaging element by controlling oil pressure, and the oil pressure switching means for switching the oil pressure by the valve to that by the lock-up clutch or the specific frictional engaging element. The engaging force of the lock-up clutch is determined to be decreased, and the engaging force of the specific frictional engaging element is determined to be increased with respect to the rise of oil pressure by the solenoid valve. The oil pressure by the solenoid valve is supplied to the lock-up clutch, and the valve produces comparatively small oil pressure in non-gear shifting and slip controlling, and the failure is determined in accordance with the state of a gear shift ratio by a failure determining means in non-gear shifting and slip controlling.