An oil pump includes: an inner rotor rotatable with a drive shaft in a unified manner on a drive-rotation axis; an outer rotor which has inner teeth configured to engage with outer teeth of the inner rotor and is rotatable about a driven axis eccentric to the drive-rotation axis; and an adjustment ring for rotatably supporting the outer rotor. A guide means which allows the adjustment ring to rotate about the driven axis, while allowing the driven axis to revolve about the drive-rotation axis, is formed of: first and second arm portions provided on the adjustment ring; and first and second guide faces with which the first and second arm portions are brought into slidable contact.