An auxiliary valve member, an auxiliary pilot pressure chamber for applying a pilot fluid pressure to the auxiliary valve body, and a transmitting rod for transmitting operating force of the auxiliary valve body to a main diaphragm are provided other than the main diaphragm which is in contact with or separating from a main valve seat by operating by means of application of the pilot fluid pressure in a diaphragm valve portion, as a mechanism for strengthening valve-closing force of the main diaphragm, so as for the main diaphragm not to be separated from the valve seat even when the fluid pressure from a load is applied to the main diaphragm as adverse pressure.