PURPOSE: To manufacture a hollow extruded material having ruggedness on the inside surface by pushing a freely rotatable knurling tool attached to the front end face of a forming use projecting part of an extruding die consisting of a combination of a female die having a forming use opening part and a male die having a forming use projecting part, against the inside surface of a hollow extruding machine.

CONSTITUTION: An extruding die 1 is a port hole die consisting of a combination of a female die 2 and a male die 3. The female die 2 has a forming use opening part 7 in an axis center part and the male die 3 has a forward protrusion flat forming use projecting part 4, and also, has plural pieces of parallel forming use unit projecting parts 5 in its tip part. To each front end face of the projecting parts 5, a knurling tool 9 is attached, and placed eccentrically near the upper part so as to come into contact in a state pushed against the upper face of the inside of a hollow extruded material. In the knurling tool 9, many projecting lines 11 being orthogonal to the extruding direction are provided on the outside peripheral surface. In such a way, the hollow extruded material having various states of ruggedness can be manufactured easily, and the heat exchanger efficiency can be improved by causing a turbulent flow in a heat exchanger body.