A vehicle oil pump having a first member and a second member relatively rotatable around one axial center such that one of them is inserted in an inner circumferential side of the other, includes: a slider member interposed there between in a direction orthogonal to the one axial center, being relatively immovable in circumferential direction around the one axial center with respect to the first member and slidable in a direction parallel to the one axial center. In the vehicle oil pump, a cam groove is formed in a circumferential surface of the second member facing the first member, a projecting portion is disposed on the slider member being fitted in the cam groove, and the cam groove causes the slider member to reciprocate in the one axial center direction in association with rotation of the slider member relative to the second member around the one axial center.