PURPOSE: To maintain a controllable gas mixture ratio, by connecting a control mixture ratio, by connecting a control gas pipeline to a gas mixture pipeline through a multi-stage solenoid valve.

CONSTITUTION: A control gas pipeline 21 is connected to a gas mixture pipeline 15 through a multi-stage solenoid valve 20 which is, in turn, controlled by a control gas change-over device 22. The control gas change-over device 22 controls the solenoid valve 20 while receiving the full-opening or full-closing of a control motor valve 8 as the electric signal from an amplifier part 6. A recorder 3 is provided with function for detecting the upper and lower limit values of a gas concn. and an electric signal when the indicator of a recording controller 3 already exceeds the set upper or lower limit control region is applied to the control gas change-over device 23 and, when said electric signal is received, the solenoid valve 20 is controlled.