PURPOSE: To reduce slide resistance while ensuring sufficient sealing function by forming a groove continuously extending on the edge face of three sections of the outer periphery edges of a vane opposed to the inner surface of a housing and inserting an U-shaped seal plate loosely in the groove.

CONSTITUTION: First and second rotary piston mechanisms 300A, 300B in charge of intake, compression and expansion, exhaust strokes rotate respectively rotors 50A, 50B synchronously about ORA in housings 20A, 20B to define respectively operation chambers M with four vanes 80. These vanes 80 are constituted from two coupled plates 84 and formed on edge endface with a groove 83 extending along three sections of the outer periphery. A seal plate 82 longitudinally divided is loosely fitted into the groove 83 at the U-shaped position corresponding to the three sections of the outer periphery of vane 80 and the inner peripheral surfaces of housings 20A, 20B. Thus, the slide resistance is reduced while a high sealing effect can be displayed.