PURPOSE: To form a cross-linkable foamed article which is foamed up to the inside of thickness, having a skin layer and is capable of having high foaming magnification, by applying back pressure higher than the atmospheric pressure to the inside of a lengthy land die by providing a cooling die at an outlet of said lengthy land die.

CONSTITUTION: A matter obtained by mixing a bridgable thermoplastic resin, a bridging agent having a higher thermolytic temperature than a plasticizing temperature of said resin and a foaming agent having a higher thermolytic temperature than that of the bridging agent is extruded within lengthy land dies 2W4 whose length is 50cm or more. The extruded mixture is made into viscosity suitable for foaming by making it unite by bridging substantially at the forestage 2 of the lengthy land die. Then the extruded mixture is made to foam by heating the same more than the thermolytic temperature of the foaming agent at rear stage parts 3, 4. A foamed article is cooled by providing a cooling die 5 at the final end of the dice where foaming is completed, back pressure is applied to the forestage part 2 through wall face resistance between the dice and the foamed article, partial generation of a bubble partially decomposing and foaming agents is prevented and a skin layer is formed.