PURPOSE: To improve efficiency of a pump, while its good workability of assembly is ensured, with clearance control of a drive gear by rotating it by a gear drive shaft providing only a function of transmitting drive power with a gear supporting function provided in a pump housing.

CONSTITUTION: A drive gear 41 and a gear drive shaft 43, engaged only in a direction of rotation, can be displaced in a rotary shaft direction and a diametric direction by meshing an engaging pawl 41a of the drive gear 41 with an engaging notch 43a of the gear drive shaft 43. That is, because of connecting structure having only a function of transmitting drive power, looseness for the drive gear 41 is absorbed by this connecting structure eliminating the looseness, so that a seal clearance can be small set of an external gear pump 4 improving pump efficiency. While because the drive gear 41 is supported to a shaft part 40a of a pump housing 40, the axial center of the drive gear 41 and the axial center of a seal surface are aligned for the pump housing 40, so that the alignment can be easily further high accurately performed by a dimensional control at manufacture time.