A polyisocyanate foam having isotropic cells is manufactured by a foaming operation in an apparatus comprising a mold composed of side walls capable of extending and returning, or folding and unfolding, in directions perpendicular to the free rising direction. Mold extending means are provided to spread out the side walls of the mold perpendicular to the free rising direction. While the foamable composition is in a creamy state, it is allowed to rise freely in one direction (the free rising direction), while the side walls of the mold are in their folded condition, until a height detecting means is actuated. Then the side walls of the mold are unfolded to enlarge the cross-sectional area of the mold so that the foamable composition is then permitted further to expand freely in directions perpendicular to the initial free rising direction, until the foamable composition is transformed from the creamy state to the tack-free state, whereby a polyisocyanate foam product having essentially spherical isotropic cells is obtained.