Gas flow with surface reaction in porous media appears in various regions of engineering. In porous media with holes as small as a molecular mean free path, Kn of gas flow in the narrow channel is on the order of unity. Therefore, the direct simulation Monte Carlo (DSMC) method is suitable to solve transport phenomena in such kind of porous media. We perform 2D DSMC simulations of such a flow. The shape of narrow channel in porous media is complicated. To reduce complexity, we propose the simplification for porous structures by cubes and polyhedra. Results for the simplification by polyhedra agree well with the result obtained in the case without simplification. Results for the simplification by cubes also show good agreement with the result without simplification when surface reaction probability is modified.