A numerical and theoretical investigation were carried out to find the possibility of appearance of a regular regime (RR) curve on the drying with a constant material temperature of a hygroscopic porous slab with mass-transfer resistance on the slab surface. The investigation afforded the following results: first, in the case of constant resistance a RR curve appeared with the same axi parameters as in the case of no resistance, while no curve appeared in the case of increasing resistance. Second, when the dependence of the moisture transfer coefficient on the moisture content is known, the relation between drying rate and mean moisture content can be predicted for the two cases of resistance from the drying characteristic function derived from the flux ratio method.