An oil pump comprising: an inner rotor provided in a casing and having outer teeth, the inner rotor being configured to be driven about a drive-rotation axis; an outer rotor provided in a casing and having more inner teeth (13 A) than a number of the outer teeth of the inner rotor, the outer rotor being configured to engage with the inner rotor in an eccentric state; a suction inlet and a discharge outlet configured to suck and discharge a fluid, each provided in the casing so as to face a space between the outer teeth and the inner teeth whose volume is changed in accordance with driven-rotation of the inner rotor; and an adjustment ring provided in the casing, the adjustment ring being coaxially and relatively rotatably fitted on the outer rotor and configured to allow a rotation center of the outer rotor to revolve about a rotation center of the inner rotor, the adjustment ring being provided with an operation portion to which a driving force is input; and a guide means which is formed in the adjustment ring and the casing and configured to guide the adjustment ring when the operation portion is operated, wherein the guide means comprises: a guide pin which is provided on one of the adjustment ring and the casing and arranged in parallel with the drive-rotation axis; and a guide groove which is provided on the other of the adjustment ring and the casing, formed along an actuation trajectory of the adjustment ring at a position opposing the guide pin, and configured to guide the guide pin, characterized in that the guide groove is formed of: a revolution guide groove portion configured to guide the adjustment ring such that the rotation center of the outer rotor is revolved along a trajectory of revolution about the rotation center of the inner rotor. an inner rotor provided in a casing and having outer teeth, the inner rotor being configured to be driven about a drive-rotation axis; an outer rotor provided in a casing and having more inner teeth (13 A) than a number of the outer teeth of the inner rotor, the outer rotor being configured to engage with the inner rotor in an eccentric state; a suction inlet and a discharge outlet configured to suck and discharge a fluid, each provided in the casing so as to face a space between the outer teeth and the inner teeth whose volume is changed in accordance with driven-rotation of the inner rotor; and an adjustment ring provided in the casing, the adjustment ring being coaxially and relatively rotatably fitted on the outer rotor and configured to allow a rotation center of the outer rotor to revolve about a rotation center of the inner rotor, the adjustment ring being provided with an operation portion to which a driving force is input; and a guide means which is formed in the adjustment ring and the casing and configured to guide the adjustment ring when the operation portion is operated, wherein the guide means comprises: a guide pin which is provided on one of the adjustment ring and the casing and arranged in parallel with the drive-rotation axis; and a guide groove which is provided on the other of the adjustment ring and the casing, formed along an actuation trajectory of the adjustment ring at a position opposing the guide pin, and configured to guide the guide pin, a guide pin which is provided on one of the adjustment ring and the casing and arranged in parallel with the drive-rotation axis; and a guide groove which is provided on the other of the adjustment ring and the casing, formed along an actuation trajectory of the adjustment ring at a position opposing the guide pin, and configured to guide the guide pin, a revolution guide groove portion configured to guide the adjustment ring such that the rotation center of the outer rotor is revolved along a trajectory of revolution about the rotation center of the inner rotor.