PURPOSE: To prevent a speed energy from being biased and restrict an occurrence of noise by a method wherein a discharging side partition starting point and a partition completion point are connected by an arc having a smaller radius than an outer circumferential radius of an air passage and projected toward the air passage.

CONSTITUTION: An annular passage having a semi-circular section is formed from a suction air passage 106 to a discharged air passage 107 under a combination of a fixed main body 108 and a rotor main body 109 A partition 105 is disposed between a suction port 103 and a discharging port 104. In addition, at the discharging side, a part ranging from a point A to a point E corresponds to an air inlet of a rotor vane and another part ranging from the point E to a point B corresponds to an air flowing-out part. In this case, the discharging partition starting point at the partition part 105 is set as an air passage inner circumferential part, a partition completing point is applied as the air passage outer circumference. A part between the two points is connected by an arc with a smaller radius (r) than a radius (R) of an outer circumference of the air passage and it is projected toward the air passage. With such an arrangement, at first an air inlet port for the air chamber 110 is closed at the parts A to E and then the air flowing-out part for the air chamber 110 is closed at the parts E to B.