An analysis for adsorbed gas on wall surfaces using an electron‐impact desorption (EID) technique was developed and applied to the evaluation of wall conditioning in the GAMMA 10 tandem mirror. The partial pressure rise due to EID (ΔPEID) was measured with a quadrupole mass analyzer installed near an electron emitting filament inserted in the central cell of GAMMA 10. The analysis using this method was performed during the wall‐conditioning period and the ΔPEIDrelated to light impurities (M=15, 28, 44) was observed to decrease by an order of magnitude with the progress of the wall conditioning. It was also observed that a good correlation exists between the ΔPEIDand the partial pressure rise induced by the plasma discharge (ΔPPID). The desorption efficiency of typically adsorbed gas molecules ξ was estimated by a simple balance equation for the partial pressures. A significant reduction of ξ due to the wall conditioning was observed.