To ensure a good assembly workability to the housing of an inner rotor without giving an effect to a seal width for the housing of the inner rotor, in an oil pump.

An oil pump is provided with an inner rotor to which a rotation is transmitted through at least two projections 21c projected and installed toward a diameter inside in the inner hole 21b in which a rotary shaft 32 is inserted and whose both side surfaces 21e, 21f are slide-contacted to the housing inside surfaces 11r, 12b, and an outer rotor 22 in which a pump room R is formed between both teeth by having the internal teeth 22a meshed with the external teeth 21a of the inner rotor and a pump room R is formed between both teeth and which is stored rotatably by off-centering for the inner rotor in the recessed part 11s of the housing. Projection parts 21c1 projected in the axial direction and inserted in the inner hole 11a of the housing are installed respectively on one side surface of respective projection parts 21c of the inner rotor and the diameter between the outer periphery surfaces of these respective projections 21c1 is set so as not to slide-contacted to the inner hole 11a of the housing at the rotation time of the inner rotor.