An oil pump is provided which includes a cam ring made of a material having a first line expansion coefficient, a pump element rotatably disposed in the cam ring and smaller in an axial length than the cam ring, a first housing member disposed on one of axially opposite sides of the cam ring and made of a material having a second line expansion coefficient larger than the first line expansion coefficient, and a second housing member disposed on the other of the axially opposite sides of the cam ring and made of a material having a third line expansion coefficient than the first line expansion coefficient.