A pressure-reducing valve is provided in which a diaphragm rod connected to a valve body of a valve mechanism is connected to a central part of a diaphragm having its peripheral edge part clamped between a body and a diaphragm cover, wherein the diaphragm (28) is formed from a disk-shaped rubber sheet for which the sheet thickness of at least its peripheral edge part is uniform in the natural state, and in a state in which the diaphragm cover (17) is fixed to the body (16) so that the peripheral edge part of the diaphragm (28) is compressively held between a collar portion (17b) of the diaphragm cover (17) and a clamping face (91) of the body (16), a portion having, of the gap between the collar portion (17b) and the clamping face (91), a minimum gap is positioned further inward in the radial direction of the diaphragm (28) than a portion having a maximum gap. This enables the sealing properties and the pull-out resistance for the diaphragm to be guaranteed while achieving a reduction in weight and a reduction in cost.