To provide a variable ring gear pump which can attain a short suppressing stroke and prompt reaction.

In this infinitely variable ring gear pump where a difference in the number of engaging teeth between an inner rotor 3 and an outer rotor 4 is 1, a plurality of extendable delivery cells 7 of tooth form shaped which is sealed with each other by respective ones are formed, and low-pressure and high-pressure ports 8, 9 of kidney shape in a cross direction are disposed in a casing by being separated from each other by webs 10, 11. The angular position of the eccentric shaft of a ring gear operating set 5 (eccentricity 17) is variable against the casing. It is preferable that the support 2 of the outer rotor 4 of the set is conducted by the outside diameter 13 of the outer rotor in an adjusting ring 14 of the same width. The adjusting ring 14 is rotatable on an inner circumference or a pitch circle 16 at a zero slip by an outer circumference or a pitch circle 15. It is formed so that a diametrical difference between the two circumferences or the pitch circles 15, 16 is twice as much as the eccentricity 17 of the ring gear operating set.