To stabilize an adjusted pressure even when the flow rate of gas is increased.

When the flow rate of gas is small, a low pressure diaphragm 75 is displaced according to the pressure change of a low pressure reducing chamber 73, and a low pressure valve 89 is closed so that the pressure of the low pressure reducing chamber 73 can be adjusted. At that time, a pressure difference is hardly generated between the upstream and downstream of an orifice 99, and a low pressure diaphragm 115 of a low pressure reducing part 113 for gas supply is displaced so that a by-pass valve 143 is closed. When the flow rate of gas is increased, a pressure loss is generated between the upstream and downstream of the orifice 99, and the pressure of a gas introducing space 127 at the downstream side is decreased, and the low pressure diaphragm 115 for gas supply is displaced according to a pressure difference between the pressure decreasing site and a low pressure reducing chamber 129 for gas supply so that the by-pass valve 143 is opened, and that a by-pass nozzle part 145 is opened. When the by-pass nozzle part 145 is opened, gas flowing out of this part can be made linear toward a gas exit passage 57.