<p>The sensitivity of the ASDEX type ionization gauge (AIG) in the mixture of argon and hydrogen gas, which is a candidate of radiator gas in the radiative divertor, is studied. In a small vacuum chamber, the sensitivity of the AIG is calibrated against capacitance-manometers, which have constant sensitivity for all gas species. Increase of the output signal AIG was observed in the mixture of 25% Ar and 75% H₂, although the actual sensitivity of the gauge against gas pressure is degraded. In the gas pressure larger than 0.8 Pa, the degradation of the sensitivity will be the main concern of the measurements. The results indicated that the molecular ions produced by the collision between the metastable argon atom and hydrogen molecule is the main cause of the change of the sensitivity. The change of sensitivity is analyzed by varying the collision energy of the electrons and the ionization cross-section of the ArH molecular ion is evaluated.</p>