A pump housing has a main oil passage, a return oil passage provided substantially in parallel with the main oil passage, and a valve insertion hole extending across the main oil passage such that a closed fore end of the valve insertion hole reaches in the vicinity of the return oil passage. The valve insertion hole has a female screw formed at an open end thereof, and a through-hole is formed in the pump housing, having one end connected to the valve insertion hole at a portion adjacent to the closed fore end thereof and an opposite end opened to the outside of the pump housing. With the valve insertion hole thus provided, a temperature sensitive valve mechanism can be readily inserted in the valve insertion hole at any time.