Gas response of semiconductor film devices to reducing gas under control of gas diffusion and reaction effects was investigated in the light of recently developed theory of receptor function of small semiconductor crystals. The conventional gas diffusion and reaction equation is applicable consistently to the analysis of this issue provided that the size of the crystals and the partial pressure of reducing gas are small enough. Under this condition, gas response can be estimated as a function of film thickness and operating temperature. The bell-shaped correlations between response and temperature as well as depth dependence of the response are simulated and discussed.