A control valve (70) is constructed so that an operating oil can be confined in a space (S). Therefore, when a lockup clutch (16) is made to stand by under the condition that a secondary pressure is input to both engaging oil chamber (11a) and releasing oil chamber (11b) when the lockup clutch (16) is changed from an off-state to an on-state, even if the lockup clutch (16) is pushed into the side of the releasing oil chamber (11b) by a centrifugal oil pressure action and a feed-back force exceeding the secondary pressure acts on a spool (74), the operating oil confined in the space (S) acts as a resistance force, so that the lockup clutch (16) is prevented from being excessively pushed into and the operating oil in the releasing oil chamber (11b) is prevented from being discharged. An engaging shock of the lockup clutch (16) can be therefore suppressed.