To surely release a lock-up clutch even in an idling state of an engine when the quantity of operating oil from an oil pump is lowered.

The torque converter 14 has a pump impeller 30 connected with a crank shaft 15 through a front cover 41, and a turbine runner 31 connected with a gear change input shaft 16 through a turbine hub 44. The lock-up piston 50 is axially movably built in the turbine hub 44, and the lock-up clutch 33 is released by controlling the supply of hydraulic oil to a release chamber 43a sectioned into the front cover 41 and the lock-up piston 50. The turbine hub 44 is provided with an electromagnet 60 attracting the lock-up piston 50, and the lock-up clutch 33 is released by magnetic force of electromagnet 60 generated by distribution of electricity in idling of the engine accompanying the lowering of the quantity of hydraulic oil.