Abstract: An oil pump of the present invention includes a housing (10, 20), a rotary shaft (30) which is supported by the housing, an inner rotor (71, 81) which is rotated in the housing integrally with the rotary shaft, an outer rotor (72, 82) which is rotated in the housing as being interlocked with the inner rotor, a rotor case (40) which is fitted into the housing and which contains the inner rotor and the outer rotor and supports an outer circumferential face of the outer rotor in a slidable manner, a side plate (50) which is arranged to be in contact with at least one annular end face of the rotor case, and an elastic member (60) which exerts an urging force to press the side plate to the annular end face of the rotor case. According to the above, slide resistance and operational torque can be reduced and side clearance can be maintained at constant.