A flow regulating device 1 is provided which includes: a valve element 19 which is movable into contact with or away from a valve seat 18; a spring 21 for exerting an urging force on the valve element 19; a diaphragm 20 fixed to an end of the valve element 19, the diaphragm having a fluid contact surface 20a and a back surface 20b; a pressure chamber 24 into which compressed air is externally introduced; and a piston 25 for transmitting the pressure of the compressed air that is introduced into the pressure chamber 24 to the back surface 20b of the diaphragm 20, wherein a portion of the piston 25 which portion receives the pressure of the compressed air has an area equal to or more than twice the area of the fluid contact surface 20a of the diaphragm 20.