The present invention relates to a method for reducing the HTO concentration in a tritium-containing aqueous solution. The present invention includes bringing water vapor or the like of a tritium-containing aqueous solution into contact with a porous material having pores in a pore diameter range of 500 Å or less, selectively occluding the HTO in the tritium-containing aqueous solution in the porous material, and obtaining a tritium-containing aqueous solution in which the HTO concentration thereof is reduced. The present invention relates to a device used for reducing the HTO concentration in a tritium-containing aqueous solution. The present invention includes a reservoir for a raw tritium-containing aqueous solution, a means for generating water vapor or the like of the tritium-containing aqueous solution, an occlusion means in which is accommodated a porous material having pores in a pore diameter range of 500 Å or less, and a means for recovering the tritium-containing aqueous solution in which the HTO concentration is reduced. The present invention furthermore includes a transfer means for transferring the water vapor or the like to the occlusion means, and a means for transferring the tritium-containing aqueous solution in which the HTO concentration is reduced from the occlusion means to a recovery means.