To provide a back filling method of a hollow section capable of carrying out a back filling of the hollow section by simple work at a low cost.

The back filling method is so constituted that crushed stones S are consecutively jetted out by pressurized air from the insertion end side of a pipe member 14 after the pipe member 14 is inserted in the hollow section 3 in the direction of an axis in a movable manner, the crushed stones S become a mass while cutting the crushed stones into each other by wedge operation from the wall surface on the depth side of the hollow section 3 to fill the hollow section with the crushed stones up to this side thereof, and a space of the hollow section 3 is filled with the crushed stones S packed in each other by the wedge operation getting along without making any unnecessary gap.