To provide a relief valve having a simple structure, possible to be manufactured at low cost from a lessened number of parts, capable of moving smoothly and reliably, and provided with excellent installation property.

In the relief valve for a filtration apparatus having a valve port 25a at an approximately center position in the bottom part side of a filtration element housed in a casing 11, a leaf spring 24 extended to the valve port 25a from a leg part 23 is formed in a presser plate 22 having the leg part 23 supporting the filtration element 1 in the bottom part of the casing. A valve body 27 is joined to the periphery of the end part of the leaf spring 24 and the spring force of the leaf spring 24 is applied to the valve body 27 to open or close the valve port 25a. The leg part 23 and the leaf spring 24 are unitedly formed by punching and pressing processing of a single sheet of presser plate 22.