PURPOSE: To improve the mixing at an foaming material injection section and carry out high grade foaming resin coating by making screw channel deeper than other sections and forming many projections of the height almost the same as that of a screw flight on the downstream position of an injection inlet in the radial direction.

CONSTITUTION: Recessed sections facing a number of recessed sections on the inner periphery of a cylinder of a screw cylinder mixing section for kneading resin having received a foaming agent or gas from an injection section are formed adjacent to a cavity 4 disposed on the screw peripheral face without screw groove and a multilayer section 3 provided with notches on a multi-line screw flight in the reverse screwing direction. A cavity 4 is interposed among the multi-line section 3, and a large number of projections 6 same as or slightly lower than the height of a screw flight 5 are formed in a deep screw channel of the injection section 1 in the radial direction. An injected foaming material, therefore, is kneaded with the resin immediately by the projections thus formed and collected to improve kneading.