PURPOSE: To greatly increase a bending resistance force of the freezed ground so as to the thickness of the freezed ground (freezed area) and to restrain the ground from being expanded by freezing.

CONSTITUTION: Freezing pipes 4, 14 and grid-like reinforcing members 13 are laid in grooves 2, 12, and thereafter, nonfreezable materials 5, 15 are filled in the grooves around the freezing pipes and the reinforcing members. At least the non freezable materials are freezed by the freezing pipes. A water absorbable polymer can be mixed in the nonfreezable materials 5, 15. A low strength part 17 which has a strength lower than that of the reinforcing members 13 may be provided in a part of each reinforcing members having a longitudinal predetermined length.