To reduce sliding frictional resistance sharply by arranging a guide means which consists of a guide groove for guiding rotation of a rotor and a guide ball which rolls in the guide groove, between the rotor and a cover or a housing.
A guide means for rotary-guiding a rotor 5 along a trochoid curve 6 is arranged between a cover 2 and the rotor 5, which consists of an endless guide groove 8 formed on the outer peripheral surface of the inner side surface 2b of a cover 2, three holding grooves 17a to 17c formed on one side surface 5e of the cover inner side surface 2b side of the rotor 5, and a guide ball 9 which is held in the holding grooves 17a to 17c freely to roll. When a driving shaft 4 is rotated and driven through a driving pulley 12, an eccentric collar 10 is also synchronously rotated so as to transmit turning effort to the rotor 5 through the outer peripheral surface and a rotor inner circumferential surface 5d. The rotor 5 is guided smoothly while rolling each guide ball 9 of the guide means in the guide groove 8 so as to rotatably operate along the trochoid curve 6.