MULTISTAGE INTERNAL GEAR PUMP

PROBLEM TO BE SOLVED: To improve volume efficiency by integrally manufacturing a driving shaft and at least a high pressure side external teeth gear in a multistage internal gear pump communicating a delivery port of a low pressure side pressurizing space with a suction port of a high pressure side pressurizing space. SOLUTION: In a high pressure pump part 13, a second external teeth gear 53 is integrally manufactured on a driven shaft 12 differently from a low pressure pump part 11, so that there is no key 39 existing in the low pressure pump part 11. There is no need to form a clearance groove of a key in a peripheral part of a driving shaft in side surface-opposed both side plates 21b, 21c of the external teeth gear. Thus, the length L of second oiltight clearance 59 formed between a side surface of the second external teeth gear 53 and both side plates 21b, 21c can be lengthened up to reaching the periphery of a driven shaft 12 from the tooth tip of the second external teeth gear 53 to lengthen by quantity of a key groove. Since a difference in pressure acting on the second oiltight clearance becomes large in the high pressure pump part 13, a leakage effect can be reduced to improve volume efficiency.