Abstract: An open-cell foam of a cross-linked polyolefin possessing uniform physical properties, a high degree of expansion, and a high open-cell ratio can be produced with high operational efficiency and high material utilization by a method which includes thermally shaping under pressure a foamable and cross-linkable polyolefin composition capable of being heated with a microwave, then heating the composition by irradiation with a microwave thereby inducing partial decomposition of a cross-linking agent and a foaming agent contained in the composition, subsequently externally heating under normal pressure the resultant intermediate partially foamed product thereby effecting decomposition of the remainders of the cross-linking agent and foaming agent, and thereafter subjecting the resultant foam to mechanical deformation thereby establishing intercommunication among the cells.