PURPOSE: To reduce the extent of noise by forming each blade of respective rotors into a notched form from the midway of a side face to the tip, and making a liquid between side faces of both advancing and delaying side blades so as to flow into a space at the discharge side or from a space at the inlet side.

CONSTITUTION: Each blade inner side 1 of respective rotors is notched from the midway to the tip, forming them into each of lines C1', C2', C3' and C4'. An interval between side faces of blades comes nearer as far as a portion for this notch so that a closing space between blades becomes smaller. In addition, discharge side space parts 13, 13 leading to the discharge side of a pump are formed in fixed centers 5, 5' via connecting passages 14, 14', forming discharge side connecting ports 12, 12' there. Therefore a fluid in a space (actuating chamber 7) held between two rotors flows into the discharge side space part 13 via the connecting port 12. Thus each clearance of these blades is contracted, decreasing the closing space, and discharge resistance of the fluid in this space is decreased, thus an energy source of noise becomes smaller so that the noise is exceedingly lessened.