The present invention relates to a method for treating tritium water-containing raw water, the method including supplying a part of raw water containing tritium water and alkali water to a circulation tank, mixing the raw water with alkali water in the circulation tank to obtain an electrolyte adjusted so as to have a desired alkali concentration, and continuously electrolyzing the electrolyte while circulating the electrolyte, thereby subjecting the raw water stored in the storage tank to alkali water electrolysis and thus gasifying the raw water. According to the invention, by gasifying tritium water-containing raw water by alkali water electrolysis, the tritium concentration in a tritium-containing hydrogen gas is diluted to 1/1,244 and the tritium water-containing raw water can be reduced in volume.