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The population inversion between the level and the level for He-Ne gaseous laser is calculated by solving rate equations in the four-level system in the steady state. Using this result, the unsaturated gain and the output power in laser oscillation as a function of discharge current are obtained for laser mediums with inhomogeneously broadened lines. The output power dependency on the excitation parameter is also obtained. The collision width and various transition rates in the transfer from helium to neon, spontaneous emission, induced emission and absorption, electron collision, and diffusion are appropriately chosen for the mixture of 1.0 torr helium and 0.1 torr neon in a 6-mm ID laser tube. The theoretical values of single-pass unsaturated gain for 65-cm plasma length and output power are 4.7 percent and 38 mW/cm2, respectively, for a discharge current of 30 mA. The experimental values are 4.5 percent and 33 mW/cm2, respectively, and almost coincide with the theoretical values. Furthermore, the saturation parameter is obtained theoretically as 490 mW/cm2.