An oil pump comprising: a rotatable inner rotor including a vane-housing unit that houses a plurality of vanes such that the plurality of vanes is slidable in a radial direction; a rotatable annular outer rotor including a plurality of vane-connecting parts (21e, 21j) connected with tip ends of the plurality of vanes located radially outward; a first volume-changing part, which is provided between the inner rotor and the outer rotor, a first volume of which is changed in response to eccentricity of the inner rotor with respect to the outer rotor, thereby providing a pumping function; and a second volume-changing part, which is provided in the outer rotor, a second volume of which is changed by a change in a distance between the vane-connecting parts (21e, 21j) adjacent to each other in a circumferential direction in response to the eccentricity of the inner rotor with respect to the outer rotor, thereby providing a pumping function, characterised in that the outer rotor includes a plurality of outer rotor pieces annularly connected to each other, each of the plurality of outer rotor pieces includes a first engaging part provided in a first end surface in an axial direction and a second engaging part provided in a second end surface in the axial direction and being capable of engaging with the first engaging part of an adjacent one of the outer rotor pieces, and the plurality of outer rotor pieces is circumferentially arranged in a state where the first engaging part and the second engaging part of the outer rotor pieces being adjacent to each other engage with each other such that a distance therebetween in the circumferential direction is variable. a rotatable inner rotor including a vane-housing unit that houses a plurality of vanes such that the plurality of vanes is slidable in a radial direction; a rotatable annular outer rotor including a plurality of vane-connecting parts (21e, 21j) connected with tip ends of the plurality of vanes located radially outward; a first volume-changing part, which is provided between the inner rotor and the outer rotor, a first volume of which is changed in response to eccentricity of the inner rotor with respect to the outer rotor, thereby providing a pumping function; and a second volume-changing part, which is provided in the outer rotor, a second volume of which is changed by a change in a distance between the vane-connecting parts (21e, 21j) adjacent to each other in a circumferential direction in response to the eccentricity of the inner rotor with respect to the outer rotor, thereby providing a pumping function, characterised in that the outer rotor includes a plurality of outer rotor pieces annularly connected to each other, each of the plurality of outer rotor pieces includes a first engaging part provided in a first end surface in an axial direction and a second engaging part provided in a second end surface in the axial direction and being capable of engaging with the first engaging part of an adjacent one of the outer rotor pieces, and the plurality of outer rotor pieces is circumferentially arranged in a state where the first engaging part and the second engaging part of the outer rotor pieces being adjacent to each other engage with each other such that a distance therebetween in the circumferential direction is variable.