PURPOSE: To stabilize a flow of liquid inside a tube by enlarging in advance the inner diameter of the part to be compressed towards the center at the inside of a tube rather than the inner diameter of a non-working part and equalizing both inner diameters after working.

CONSTITUTION: A working device of tubular material for plastic work, etc. which give a compression force to the radius direction of a tube is formed by a detecting means 19 of inner diameter, a detecting means 23 of feed speed, an arithmetic device 25 for calculating an arrival time of the tubular material 11 reaching the position of a work tool 18 and a control device 27 for driving the work tool 18. And the inner diameter D4 of a working part 11a is made larger than the inner diameter D5 of a non-working part 11b in advance and the inner diameters of the worked part 11a and unworked part 11b after plastic working are equalized.