To provide an underground space part filling method low in cost and easy to execute.

The underground space part filling method has steps of compressing a ground material of a hard urethane foam to mold briquettes 1, packing a plurality of briquettes 1 in a net bag 3 to form a briquette assembly 2, filling a plurality of briquette assemblies 2 in an underground space part 4, and spraying a foam resin undiluted solution to the filled briquette assemblies 2 to fix them. Since a waste material or a ground material of the waste material can be used as the ground material of the hard urethane foam, cost is low, and since the briquettes 1 are packed in the net bag 3 to form the briquette assemblies 2 and then the briquette assemblies 2 are filled in the underground space part 4 to spray the foam resin diluted solution, construction is easy compared with a conventional filling method.