[en] High potentiality leading to rapid cooling of a high temperature plasma is investigated in a gas contact cooling method in which the effect of finite time for mixing the plasma and the contact gas is considered. The calculation has shown that the cooling is much more rapid than the radiative loss cooling and that the gain per unit length of the HeII 164 nm is ∼2 cm^-1 for the laser oscillation under an optimum condition. Possibility of realizing a shorter wavelength laser is also discussed in a plasma with higher charge number Z. (author)