In a driving zone where a torque converter is to be locked up, a lockup force of a lockup clutch is weakened in the deceleration driving state compared with that in the non-deceleration driving state. Moreover, during transition from the deceleration driving state to the non-deceleration driving state, increase control of the lockup force is forcedly delayed for a predetermined time. By this, during acceleration from the deceleration driving state, the clutch is made to slip with a rapid increase in engine output torque and a rise in the engine output torque is buffered by the lockup clutch.