To provide a method for adding water after filling a filler by a powder filling method capable of filling premixable powder in a cavity, even if there is a specific gravity difference, in which buoyancy does not operate.

A basic method humidifies the powder by imparting liquid to the powder after fitting the dried powder in the cavity. A concrete first method supplies the liquid by arranging a liquid supply pipe having an orifice for dripping the liquid in advance in the cavity. A second method supplies the liquid by filling the dried powder after storing the liquid in advance in a lower part in the cavity. A third method wets the powder by supplying moisture by the inflow of underground water or rainwater after filling the dried power in the cavity existing under the ground. The powder selectively adopts powder mixed with cement powder and crushed stone fine powder or powder of mixing the cement powder with fly ash.