PURPOSE: To prevent the occurrence of cavitation phenomenon, by a method wherein an opening, protruding in the direction of rotation of a rotor, is additionally formed on the terminal of a suction port, thus increasing in the area of a fluid inlet formed right before a space between an inner and an outer rotor is closed.

CONSTITUTION: A suction port 2 has an opening 11 which is additionally formed on the terminal in the direction of rotation of a rotor and in a part of a straight line 6 between a small circular surface 4 and a large circular surface 5. Thus, a fluid inlet area S'1 right before a space 10 is closed is remarkably increased as compared with the previous inlet area S1, and fluid smoothly flows in the space 10, resulting in prevention of the occurrence of a cavitation phenomenon.