PURPOSE: To ensure that a gap is created between a mandrel and a tubular body and thereby the tubular body can be easily drawn from the mandrel by introducing a trimming die into the inner surface of the tubular body and allowing a compressed air to work in an air-tight state between the trimming die and the mandrel.

CONSTITUTION: A holder 7 is provided in a socket 4 on a main chuck system 1, and a tubular body 3 inserted into a mandrel 2 is introduced into the socket 4 to hold it. Next, the tubular body 3 is allowed to extend with the concurrent action to move a trimming die 19 formed in the main chuck system 1 forward, and the trimming die 19 is thrusted into the interior of the tubular body 3. Then a compressed air is fed from the tip of the trimming die 19 to create a gap between the mandrel 2 and the tubular body 3. After that, the tubular body 3 is drawn by moving the main chuck system 1 or the mandrel 2. The trimming die 19 is fitted into a cylinder 6 formed in the main chuck system 1 in such a manner that the die 19 is slidable on a cylinder 6, and the main chuck system 1 is preferably supported by a support shaft through a universal coupling.