Internal gear pump

An internal gear pump comprising:

• an inner rotor;
• an outer rotor that rotates by having predetermined eccentricity to a rotation center of the inner rotor;
• an outer ring that includes a holding-inner peripheral part which holds the outer rotor in a freely rotatable state, and at least three cam protruded parts formed along a circumferential direction of an outer periphery surface of the outer ring;
• a pump housing that has a rotor chamber in which the outer ring is freely oscillatably arranged;
• protruded-wall surface parts that are formed on an inner periphery side surface of the rotor chamber, are formed in the same number as that of the cam protruded parts, and are always brought into contact with the cam protruded parts;
• and
• operation means for oscillating the outer ring, wherein
• positions of the protruded-wall surface parts are set so that a diameter center of the holding-inner peripheral part of the outer ring is moved by the operation means along a locus of a circle of which a radius is the eccentricity to the rotation center of the inner rotor.