The present invention relates to an oil pump for use in and attached to a device (e.g., chain saw) having an oil-supplied portion requiring an oil supply. An oil pump supplies lubricating oil to a guide groove of a guide bar in such a manner that a plunger having a gear portion meshing with a drive worm provided on a chain saw reciprocally moves in a pump body while rotating. Two guide pins for positioning an oil pump when attaching the oil pump to a chain saw body are provided on an attachment surface of the pump body to the chain saw body. The two guide pins are spaced apart in an axial direction of the plunger, each disposed on a centerline of a transverse cross section of the plunger.