PURPOSE: To damp noise by intentionally and irregularly interrupting the periodic process of pump action during one rotation of a driving shaft.

CONSTITUTION: In an internal gear pump, the late closing of a chamber 2 in its turn the timing shift of pressure increase can be obtained by reducing the tooth height of an individual tooth 1, and also the same effect can be realized by making the pitches of teeth different with each other. At that time, the number of teeth of outer and inner gears 3 and 4 must be divided by an integer, consequently the same sections positioned in rolling peripheral surfaces can be constantly contacted with each other at the time of rolling. For instance in the case of 12:18, different pitches can be given to six teeth, and these pitches can be repeated two or three times in a circumference, thereby damping pump noise by intentionally and irregularly interrupting the periodical process of pump action.