Wakkanai siliceous shale (WSS) is a natural meso-porous material. This study aimed to improve its capacity to adsorb humidity. The shale's surface was supported by chlorides by being dried at 150 degrees C after impregnation with a chloride solution. The change in specific surface area, pore size distribution and the water sorption/desorption isotherm of the supported shales were investigated. From the water sorption isotherms, the amount of water sorption of WSS supported by LiCl 10% solution increased to 150 mg/g, while that of the natural WSS was 25 mg/g in the range of relative humidity from 30 to 60%. Similarly, WSS supported by NaCl adsorbed water about 7 times more water than natural WSS did. It was suggested that this improvement resulted from water transport into NaCl caused by capillary condensation of water in the WSS pores.