A flow regulating device comprising: a housing part including a flow channel formed in the housing part, and an inlet port and an outlet port provided at respective ends of the flow channel; a valve element part which is movable to get into contact with or away from a valve seat part provided in the flow channel so as to place the flow channel in a closed state or an open state; a spring part for exerting an urging force on the valve element part in a direction to bring the valve element part into contact with the valve seat part; a diaphragm fixed to an end of the valve element part, the diaphragm having a first surface which contacts with a fluid flowing in the flow channel, and a second surface which does not contact with the fluid; a pressure chamber into which an operating gas is externally introduced; a piston part for transmitting pressure of the operating gas that is introduced into the pressure chamber to the second surface of the diaphragm, wherein a portion of the piston part which portion receives the pressure of the operating gas has an area equal to or more than twice the area of the first surface of the diaphragm; and a membrane member disposed between the pressure chamber and the piston part for isolating the pressure chamber from the piston part, wherein the piston part receives the pressure of the operating gas introduced into the pressure chamber through the membrane member; characterised in that: the piston part includes a communication hole extending from a surface of the piston part which surface contacts with the membrane member to a surface of the piston part which surface contacts with the diaphragm. a housing part including a flow channel formed in the housing part, and an inlet port and an outlet port provided at respective ends of the flow channel; a valve element part which is movable to get into contact with or away from a valve seat part provided in the flow channel so as to place the flow channel in a closed state or an open state; a spring part for exerting an urging force on the valve element part in a direction to bring the valve element part into contact with the valve seat part; a diaphragm fixed to an end of the valve element part, the diaphragm having a first surface which contacts with a fluid flowing in the flow channel, and a second surface which does not contact with the fluid; a pressure chamber into which an operating gas is externally introduced; a piston part for transmitting pressure of the operating gas that is introduced into the pressure chamber to the second surface of the diaphragm, wherein a portion of the piston part which portion receives the pressure of the operating gas has an area equal to or more than twice the area of the first surface of the diaphragm; and a membrane member disposed between the pressure chamber and the piston part for isolating the pressure chamber from the piston part, wherein the piston part receives the pressure of the operating gas introduced into the pressure chamber through the membrane member; characterised in that: the piston part includes a communication hole extending from a surface of the piston part which surface contacts with the membrane member to a surface of the piston part which surface contacts with the diaphragm.