A semi-infinite expanse of a gas in contact with its plane condensed phase is considered. The steady gas flow condensing on or evaporating from the condensed phase is investigated numerically on the basis of the Boltzmann–Krook–Welander equation in the case where there is a gas motion along the condensed phase. First, it is shown by a time-dependent analysis that the steady evaporating flow is possible only when there is no gas motion along the condensed phase. Then, with the aid of time-dependent and time-independent analyses, the effect of gas motion along the condensed phase on the steady condensing flow is clarified. That is, the relation of the parameters at infinity and of the condensed phase that allows a steady flow, together with the accurate profiles of the steady solutions, is established. The present results of the half-space problem complete the boundary condition of the compressible Euler equation required in describing steady gas flows around the condensed phase of a smooth shape in the continuum flow limit.

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