A tightening force of a lockup clutch (2) exerted between a pump impeller (1a) connected to an engine (21) and a turbine runner (1b) connected to an automatic transmission (23) is controlled by a controller (5). The controller (5) determines a target relative rotation speed of the pump impeller (1a) and the turbine runner (1b), and performs feedback control of the tightening force such that the difference between the target relative rotation speed and the real relative rotation speed is decreased. The controller (5) also performs feedforward control of the tightening force. When the variation of the relative rotation speed due to the feedforward control has exceeded the predetermined value, the controller (5) corrects the feedback control amount to moderate the effect of the variation, thereby suppressing sudden change in the tightening force.