PURPOSE: To provide an oil pump which can prevent the seizure caused by the lack of the lubrication oil between a housing and a differential which is reciprocated in the housing.

CONSTITUTION: A pump chamber 15 is formed by a hollow part formed in a differential 16 and the other end part of a cylindrical subplunger 18 where one end is fixed to the housing. An oil reservoir 19 which is formed in a circular groove shape is formed on the outer circumferential surface opposite to an oil suction hole 16a and an oil discharge hole 16b of the differential 16. In addition, an oil passage 17 is formed to communicate the pump chamber 15 with the oil reservoir 19.