First temporary capacity reduction control that when acceleration ON is judged during coasting lock-up, brings lock-up clutch (20) into slip state by decreasing torque transmission capacity of lock-up clutch and subsequently returns lock-up clutch to lock-up state by increasing torque transmission capacity is executed. Control unit performing second temporary capacity reduction control that when return of accelerator pedal depression is judged during the progress of returning to lock-up state, decreases torque transmission capacity again and subsequently returns lock-up clutch to lock-up state by increasing torque transmission capacity is provided. In second temporary capacity reduction control, torque transmission capacity is decreased with predetermined torque transmission capacity by which lock-up clutch is not fully disengaged being lower limit value. Abrupt engagement of lock-up clutch and engine racing can be therefore avoided when acceleration is changed from ON to OFF during control of lock-up clutch from disengagement or slip state to lock-up state.
