To improve the assembling property of a rotary oil pump while reducing the manufacturing cost.

This rotary oil pump 1 comprises an inner rotor 12, an outer rotor 13, and a housing 15 having a bearing 15a for a shaft 11a while housing the inner rotor 12 and outer rotor 13, the inner rotor 12 and the shaft 11a being engagingly locked by a knock pin 14. A sprocket 11b for rotating the shaft 11a is integrally formed on the outer side of the part to be held by the bearing 15a of the shaft 11a, and constituted so that the distance CL from the end surface 15b of the bearing part 15 of the housing 15 to end surface 11e of the bearing part 15a-side of the sprocket 11b is larger than the depth D of the knock pin 14 fitted to a recessed part 12c from the end surface of the inner rotor 12 in the assembled state.