To produce the titled foam of a high expansion ratio, by placing and expanding a polyolefin-containing expandable composition in the primary mol and placing and expanding (secondary expansion) the obtained intermediate product in the secondary mold. An expandable composition (A) is obtained by mixing and kneading a polyolefin (a) with a blowing agent (b) having decomposition temperature >= the melting temperature of component (a), a blowing aid (c) and a crosslinking agent having a decomposition temperature >= the flow initiation temperature of component (a) and/or a filler. This composition A is placed in a primary mold 1 comprising pressurized sealed container placed within a press 2 and its primary expansion to an expansion ratio of 2-7 is performed by heating for a definite time at an applied pressure while releasing the pressure to obtain an intermediate product B. This intermediate product B is placed in a secondary mold 3 comprising a heating container, heated for a definite time at normal pressure to decomposed the blowing agent remaining undecomposed and to effect its secondary expansion. The expanded product is heat-aged and cooled to obtain polyolefin foam C of a high expansion ratio >=40. |