To maintain the state of solid solution where the precipitation of alloy component is suppressed to room temperature and to secure precipitation quantity which is effective to impart strength by aging treatment by cooling a hollow extruded shape M just after extrusion from the inside surface.

A chill supply pipe (chill supply hose 11) is inserted into the hollow extruded shape M which is extruded from a die 1 from an opening on the outlet side of the hollow extruded shape M and a distance d from a chill jetting nozzle 12 attached at the tip of the chill supply pipe to the end face on the outlet side of the die 1 is maintained constant. By jetting the chill from the chill jetting nozzle 12 toward the opening on the outlet side from the radial direction of the hollow extruded shape M, the inside surface of the hollow extruded shape M is cooled.