To reduce weight, to reduce installing manhours, and to reduce cost by reducing the number of part items of a relief valve.

This relieve valve has a valve element 32a housed in a housing 100 energized by a spring 32c, and allowed to abut to a seat surface 102a. When applying prescribed pressure in the valve opening direction to the valve element 32a, the valve is opened, pressure is released, and the seat surface 102a is directly formed in the housing 100.