A diaphragm valve includes an outer wall section for valve seat mounting provided in a primary-side opening section, a stepped section provided on the inner periphery of the outer wall section, a seat ring press-fitted into and fixed to the stepped section, a flange section provided in the seat ring, and a press-fitting groove for valve seat mounting configured with a space between the seat ring and the outer wall section and the flange section, in which a valve seat press-fitted into and fixed to the press-fitting groove has an upper portion slightly protruding in a state where an extension section covers a position in the vicinity of a top portion of the outer wall section and an extension section the inner periphery of which is decreased in the upper portion of the valve seat covers a position in the vicinity of a top portion of the seat ring.