A rarefied gas flow through a thin slit caused by a small pressure difference is studied on the basis of the kinetic equation. The Bhatnagar–Gross–Krook model equation is solved numerically by the optimized discrete velocity method. The gas–surface interaction is assumed diffuse‐specular. It is shown that the dependence of the mass flow rate on the gas–surface interaction law is negligibly small at any Knudsen number. Analytical formulas for the flow rate embracing the whole range of the Knudsen number are obtained. ©1996 American Institute of Physics.