To easily execute adjustment of a reaction gas introduction quantity of a small flow rate.

A split passage 28 is formed by branching on the upper stream of a resistor tube 22, and flow control valves 27, 29 are installed on the upstream and the split passage 28 respectively. The second flow control valve 29 controls a split flow rate so that a detected gas pressure of a pressure gauge 24 will agree with a target pressure P1 calculated corresponding to a reaction gas introduction quantity into an ionization chamber 11. On the other hand, the first flow control valve 27 sends the reaction gas of a flow rate determined by adding a sufficient split flow rate with the reaction gas introduction quantity. As a result, the reaction gas of a desired introduction quantity f1 flows in the resistor tube 22 and the reaction gas of a most residual flow rate f2 flows in the split passage 28, and as soon as the target pressure P1 is modified, the introduction quantity is changed followingly.