PURPOSE: To prolong the life of an air seal for a rotary joint by employing a control mechanism for opening an air pressure passage to atmosphere while closing both ends of the air pressure passage of the rotary joint at a specified time when an air clutch is engaged.

CONSTITUTION: A piston 14 in an air cylinder 13 fit in a clutch cover is driven with pneumatic pressure to drive a pressure plate 16. When the clutch is engaged, only the 1st and 2nd passages 41a, 41b of a rotary joint 50 are opened to the atmosphere by the operation of a 1st and 2nd solenoid valves 61, 62 and an opening/closing valve 70. Under the condition of the clutch in engagement, load of pneumatic pressure from an air tank 33 is not applied to the 1st and 2nd passages 41a, 41b. Thus, heat and friction due to the sliding of the air seal of the rotary joint 50 are greatly reduced and the life of the air seal is remarkably prolonged.