Passages with rough walls allow for smaller conductance for rarefied gas flow than passages with smooth walls. For the purpose of clarifying the effects of surface roughness on rarefied gas flow through channels, surface roughness is modeled by circular cones with the same base radius. The cones are located in a statistical manner consistent with the measured angle distributions of actual surface roughness. Molecules are assumed to scatter on the cone surfaces according to the cosine law. The calculated results predict well the measured reduction of free-molecule and near-free molecule flow conductance through the passages composed of two flat plates.