Filling foam composition capable of providing excellent rustproofing and also suppressing reduction of a foam ratio of the filling foam composition even when reusing unwanted parts thereof and inferior goods thereof; a foam filling member using the same filling foam composition; and a filling foam produced by foaming the filling foam composition. The filling foam composition is prepared by mixing foamable polymer, a foaming agent, and a basic oxide in such a mixing ratio that a ratio of the basic oxide can be in the range of 0.05-70 parts by weight per 100 parts by weight of foaming agent. The mounting member for the filling foam composition to be mounted in an internal space of a hollow member is produced by mounting a mounting member on the filling foam composition obtained. The filling foam is formed by fitting the foam filling member in the internal space of the hollow member and, then, foaming the filling foam composition by the application of heat.