Wakkanai siliceous shale, a natural mesoporous material, is impregnated with lithium chloride, calcium chloride and sodium chloride solutions to improve its moisture adsorption capacity. However, excess impregnation leads to elution of sorbed moisture from pores to the surface under humid conditions, and the surface of the sample becomes wet. To determine appropriate concentrations of impregnating chlorides, the surface wettedness of the chloride-impregnated shale was estimated from the surface color changes of samples exposed to 75% RH air by using a digital scanner. The appropriate concentrations of lithium chloride and calcium chloride for the shale were 4 and 5 wt%, respectively. Moreover, the maximum moisture sorption of the shale impregnated with the appropriate chloride concentration agreed with the moisture retention predicted from the mesopore volume. This indicates that, in order to avoid elution of the chloride solution to the surface, the maximum moisture sorption of the chloride-impregnated shale should be smaller than the mesopore volume.