flow rate of supplied water the primary valve 4 is provided so that conduit orifices 6 provided on the top side of the secondary valve 5 for conducting the minumum quantity of gas are opened and closed in a snapping manner through the intermediary of a diskshaped constant force spring 7 the construction described above is not different specifically from the prior art water pressure operated automatic gas valve described previously the constant force spring 7 is a spring having a region in which a force is not varied for displacement in accordance with the present invention the primary valve 5 is constructed as a proportional i e modulation valve whose opening is varied in accordance with the amount of movement of the valve rod 2 a first adjusting screw 8 is threaded in the top portion of said secondary valve 5 so that it supports the constant force spring 7 the upper end of the valve rod 2 piercing through the secondary valve 5 is made to face the lower surface of said constant force spring 7 while the lower end of a second adjusting screw 17 threaded in the primary valve 4 is opposed to the upper surface of said constant force spring 7 a gap apart a space equivalent to the distance of movement of the constant force spring 7 to a neutral position in more detail the secondary valve 5 is formed to be a modulation valve by providing a tapered portion 5a in the outer.