To reduce drag torque without spoiling a torque transmitting function by arranging a changeover means in a circuit connecting a fluid pressure operating source to an operation chamber by a working fluid circulating means, and intercepting communication of the operation chamber with a fluid pressure operating source to communicate with the atmosphere.

A working fluid circulating means 6 is provided with a reserve tank 65 receiving working fluid, and working fluid in the reserve tank 65 is discharged to a passage 661 by a hydraulic pump 60. The working fluid discharged in the passage 661 is supplied to a passage 662 communicated with an operation chamber 4a through an electromagnetic changeover valve 67. The electromagnetic changeover valve 67 communicates the passage 661 with 662 at OFF, and at ON communication of the passages 661, 662 is intercepted to communicate the passage 662 with the atmosphere through a filter 68. By changeover of the electromagnetic changeover valve, the operation chamber 4a is communicated with the atmosphere, air is sucked to reduce a working fluid quantity, and drag torque can be quickly reduced.