PURPOSE: To provide control through which release of lockup of a converter is executed without a shock and with response as high as possible under any transmission torque.

CONSTITUTION: When it is decided by a controller 5 based on a throttle opening TVO and a car speed VSP that a torque converter is in a lockup release area, a differential pressure PA-PR between the front and the rear of a clutch 2 is reduced through a control valve 3 by reduction of a signal pressure PS due to the decrease in a duty D, lockup of a torque converter 1 is released through release of a clutch 2. In the release, the differential pressure between the front and the rear is set to an initial difference pressure right before the clutch 2 is slid according to input torque during discrimination. Thereafter, after a lapse of a specified time, the differential pressure is gradually decreased from the initial differential pressure to 0. The initial differential pressure causes release of lockup to a limit with high response within a range wide enough to prevent the occurrence of a shock.