Lockup control of torque converter

An engaging force of 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 engaging 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 engaging force in a increasing direction. When the engine (21) is in a predetermined engine-stall prevention condition, the controller (5) prohibits the feedforward control so as to prevent an interference between feedback control and feedforward control.