PURPOSE: To make continuous projections on the inside surface of an extruded material such as a pipe or bar, by providing grooves at a prescribed angle on the surface of a mandrel and extruding the material on the surface of the mandrel.

CONSTITUTION: A prescribed number of grooves 18 are provided on the surface of the mandrel 16 so that the grooves are spiralled at an angle of about 25° to the axial direction. The mandrel 16 is retained in an extrusion hole 15, thereby defining an annular extrusion opening 19. A billet of aluminium or the like is extruded through the inlet port 13, extrusion opening 19 and outlet port 14 of a die and a discharge hole 20 so that a pipe A is continuously shaped. At that time, the extruded material is compressed into the grooves 18. As soon as the material is passed off the mandrel, the material is released from the compression. As a result, the continuous projections are made on the inside surface of the pipe A, which can be as a heat exchange tube or the like.