Water vapor transfer within dried zone appearing during the falling drying rate period of non-hygroscopic porous slab was examined theoretically on the basis of a set of the non-isobaric unidirectional gas transfer equations that had been derived by using the equations for the one-dimensional gaseous transfer of the water vapor-air binary component system. The examination was performed under conditions corresponding to hot air drying, superheated steam drying with air leakage or vacuum drying with air leakage. As a result, the set of the strict non-isobaric unidirectional vapor transfer differential equations was successfully integrated and a set of approximate equations was obtained. Calculated results by five approximate equations, namely, the non-isobaric approximate equations, the isobaric unidirectional diffusion equation with or without the effect of the Knudsen flow and the vapor flow equation with or without the Knudsen, were compared with the results of the strict differential equations, and the applicable regions of the five approximations were clarified for the drying rate and the total pressure generated within the slab. The non-isobaric approximation showed an excellent agreement with the strict equations within extensive limits of the Surface temperature and the pore size. The isobaric approximations were effective for pores of micron size in which little total pressure was generated. The vapor flow approximations were effective for all cases of drying in which pores were filled approximately with the vapor only. Only the non-isobaric approximation was effective for all cases of drying involving sub-micron size pores filled with both vapor and air in which considerable total pressure was generated.