An oil pump (100; 200; 300; 400) comprising: a rotatable inner rotor that includes a vane-housing unit housing multiple vanes so as to be capable of sliding in a radial direction; a rotatable annular outer rotor (20; 220) that includes multiple vane-connecting parts (21h, 221h) connecting tip ends of the multiple vanes on an outside in the radial direction and that includes multiple outer rotor pieces (21; 221), each of which being provided for each of the multiple vanes and including a vane-connecting part (21h; 221h); first volume-changing parts (61; 261), which are provided between the inner rotor and the outer rotor (20; 220), and a first volume of which is changed in response to eccentricity of the inner rotor with respect to the outer rotor (20; 220), thereby providing a pumping function; and second volume-changing parts (62; 262), which are provided in the outer rotor (20; 220), and a second volume of which is changed by a change in a distance between adjacent vane-connecting parts (21h, 221h) in a circumferential direction in response to the eccentricity of the inner rotor with respect to the outer rotor (20; 220), thereby providing a pumping function, the second volume-changing parts (62; 262) which are configured to be capable of changing the second volume by the change in the distance between the multiple vane-connecting parts (21h, 221h) of the outer rotor (20; 220) in the circumferential direction by changes in radial slide positions of the tip ends of the vanes on the outside in the radial direction in response to the eccentricity of the inner rotor with respect to the outer rotor (20; 220), characterised in that the multiple outer rotor pieces (21; 221) are circumferentially arranged in a state where adjacent outer rotor pieces (21; 221) engage with each other so as to be capable of changing a distance therebetween in the circumferential direction, and the adjacent outer rotor pieces (21; 221) engage with each other in the circumferential direction while having engagement spaces (201, 202, 203) constituting the second volume changing-parts, and the second volume of the engagement spaces (201, 202, 203) is changed by a change in the distance between the adjacent outer rotor pieces (21; 221) in the circumferential direction. a rotatable inner rotor that includes a vane-housing unit housing multiple vanes so as to be capable of sliding in a radial direction; a rotatable annular outer rotor (20; 220) that includes multiple vane-connecting parts (21h, 221h) connecting tip ends of the multiple vanes on an outside in the radial direction and that includes multiple outer rotor pieces (21; 221), each of which being provided for each of the multiple vanes and including a vane-connecting part (21h; 221h); first volume-changing parts (61; 261), which are provided between the inner rotor and the outer rotor (20; 220), and a first volume of which is changed in response to eccentricity of the inner rotor with respect to the outer rotor (20; 220), thereby providing a pumping function; and second volume-changing parts (62; 262), which are provided in the outer rotor (20; 220), and a second volume of which is changed by a change in a distance between adjacent vane-connecting parts (21h, 221h) in a circumferential direction in response to the eccentricity of the inner rotor with respect to the outer rotor (20; 220), thereby providing a pumping function, the second volume-changing parts (62; 262) which are configured to be capable of changing the second volume by the change in the distance between the multiple vane-connecting parts (21h, 221h) of the outer rotor (20; 220) in the circumferential direction by changes in radial slide positions of the tip ends of the vanes on the outside in the radial direction in response to the eccentricity of the inner rotor with respect to the outer rotor (20; 220), characterised in that the multiple outer rotor pieces (21; 221) are circumferentially arranged in a state where adjacent outer rotor pieces (21; 221) engage with each other so as to be capable of changing a distance therebetween in the circumferential direction, and the adjacent outer rotor pieces (21; 221) engage with each other in the circumferential direction while having engagement spaces (201, 202, 203) constituting the second volume changing-parts, and the second volume of the engagement spaces (201, 202, 203) is changed by a change in the distance between the adjacent outer rotor pieces (21; 221) in the circumferential direction.