PURPOSE: To obtain a cable, whose insulated core is difficult to draw off from an external conductor, by making the size of air foam near the outer periphery of a foamable resin larger than the size of air foam at the central part thereof when the foamable resin is extruded, for coating, over the surface of a center conductor to provide a foamed insulated electric wire for coaxial cable.

CONSTITUTION: Pellet-like raw material resin is charged into a barrel 2 from a hopper 4 and is heated therein up to a temperature higher than the melting point. Then, a gas of foaming agent is injected from a gas injection hole 5 while the resin is being stirred by a screw 14 and a projection 16, in which condition, the gas is diffused into the resin. Next, the resulting resin is introduced into a space 28 via an extruding port 6 and a molten-resin supply port 10 and then is coated, from a coating port 20, over the outer peripheral surface of a central conductor 18 sent out from a tip end portion of a feed pipe 22. Then, the resulting conductor is cooled in a cooling tub and taken up. At this time, the temperature of the cross-head 8 as a whole and the temperature of a die 26 contained therein are controlled so as to prevent solidification of the resin. When, in this way, the resin is extruded out into the atmosphere from the coating port 20, the air foam near the outer periphery is made larger in size than that at the center.