PURPOSE: To make a wall thickness distribution of Ti constant by heating a composite billet whose sheath is Ti to a specified temperature, and thereafter, executing a hot hydrostatic pressure extrusion of an extrusion ratio exceeding a prescribed value, and thereafter, executing rerolling of a draft of a prescribed value or below, and softening annealing.

CONSTITUTION: A composite billet 1 obtained by forming an outside periphery 3 of a core member 2 of a copper material by a Ti material is haeted to 600W750°C after degassing the inside sufficiently, charged into a container 31 of a hot hydrostatic pressure extruding machine, and extruded by ≥6 extrusion ratio from a die 34 having a round hole die. To a composite extruded material 11, cold rolling of ≤60% draft and rerolling are executed and softening annealing of 550W650°C is performed. In this way, no crack is generated in an interface of the sheath and the core part, a wall thickness distribution of Ti becomes roughly constant, and a rectangular material whose sectional shape is good can be obtained.