To surely transfer control from normal lock-up control into a lock-up fastening force control at the time of decelerating driving so as to prevent engine stalling.

A shifting controller 6, a prescribed control program is carried out on the basis of a signal from a sensor group 7 to be connected and a synchronous signal from an engine controller 5 connected by a communication circuit 7, and thereby, for a period from an instant time transmitted from an accelerating driving condition or a stationary driving condition to a decelerating driving condition till an engine output becomes a negative level by operation of a driver, a lock-up fastening force is controlled by a first fastening force target value decided on the basis of the driving condition and a second fastening force target value which is the sum of an increasing one decided on the basis of an engine output. For a period from when the engine output becomes a negative level, lock-up fastening force is controlled by the fastening force target value reduced smoothly from the second fastening force target value with a prescribed decelerating speed so as to converge to the first fastening force target value.