PURPOSE: To eliminate the strain remainng in foam by prescribing the relation between the inner volumes and dimensions of primary and secondary foaming molds to a definite range in two-stage foam molding.

CONSTITUTION: At first, a foaming agent is added to a foam material to be kneaded therewith in a first stage process and, subsequently, a mold is filled with the resulting kneaded mixture and hermetically closed under predetermined pressure and heated for a predetermined time at predetermined temp. to decompose a part of the foaming agent and a part of a crosslinking agent and pressure is released at the time of heating at high temp. to take out intermediate foam from a primary foaming mold. In a second stage process, a secondary foaming mold is heated to predetermined temp. and loaded with the intermediate foam in a high temp. state to obtain final foam of low density having uniform physical properties. Herein, the dimension ratio of the primary and secondary foaming molds, that is, the expansion ratio of the intermediate foam and the final foam is set to the cube root of the multiple K of foaming magnification or the range in the vicinity of the multiple K.