To provide a void from occurring in the ground for filling between an excavation void and two U-tubes.

In an excavated hole H formed underground related to two U-tubes 11 where the lower end parts of linear outward tube 12 and return tube 13 are connected with a connection flow path 14, each of outward tube 12 and return tube 13 are arranged with almost constant intervals while both connection flow paths 14 so cross as to stack each other. A grout pipe 15 whose lower end almost reaches the bottom of excavated hole H is inserted in the central part of two arranged outward tubes 12 and return tubes 13, with the excavated hole H filled with a grout G supplied through the grout pipe 15 with no void.