PURPOSE: To prevent clamping shock caused by changes in operation states by prohibiting any change in the operation states of an auxiliary machine control means until turning into a clamped state or a released state during controlling clamping oil pressure of a lockup clutch.

CONSTITUTION: A control unit 5 receives signals from respective sensors 8-11 for oil temperature, vehicle speed, throttle opening and engine rotational speed, and form an air conditioner switch 12 sent by a driver so as to control operation of the lockup solenoid 6 based on a lockup schedule. Clamping oil pressure of a lockup clutch 1e is thus controlled and the control unit 5 prohibits, during control poriod, the control means of the air conditioner 13 to change operation state of the air conditioner 13 until it turns into a lockup clamping state or a released state. Therefore clamping shock of the lockup clutch 1e caused by change in operation state of the air conditioner 13 is prevented from occurring.