PURPOSE: To produce a hollow structure whose hollow interior is uniformly filled up with foam under wide conditions.

CONSTITUTION: In a method for filling the interior of a hollow member with foam by charging foamable particles into the hollow part of the hollow member to foam the same, as the foamable particles, non-foamed or pre-foamed particles composed of a foamable partially crosslinked vinylidene chloride resin containing a foaming agent uniformly and becoming a porous structure stabilized by a crosslinked structure at the time of foaming are used.