PURPOSE: To smoothly perform a diameter reducing by heating by installing a collect chuck structure on the outer periphery of the material to be worked and performing a plastic work by a die or roll from the external side.

CONSTITUTION: The collect chuck structure 2 having a slit 22 at its tip part 21 is set up on the outer periphery of the working stock of the pipe made of Mo, etc. and the part of its tip part 21 is reduced at its diameter by compressing its from the outside and the position of a die 3 or roll is moved as well. The plastic work of a work stock can smoothly be promoted by winding the very thin plate 5 made of a spring on the inner wall of the tip part 21 of the collect chuck structure 2. The core wire 4 of a ceramics material, etc., burying a nichrome wire, etc., inside the inner hole of the work stock is set up, the working is performed in the atmosphere of an inert gas and the heat of about 200W700°C is given. With this method the diameter reducing can smoothly be performed at the necessary temp.