PURPOSE: To perform a reducing work of pipe end diameter in a short time by small working force, by scarcely varying the circumferential length of a section of a pipe, varying its sectional shape, and reducing the maximum diameter in the section of the pipe.

CONSTITUTION: A pipe (a) is worked to a flower-shaped section (figure (b)) by pressing it from four directions centering around the section by the first opposed roll dies 4 of two pairs. Subsequently, the flower-shaped section pipe shown in the figure (b) is folded to a shape whose corner section is reduced, by pressing it from a position turned by 45° from said roll dies by other roll dies 5, as shown in the figure (c). In this case, the die 4 forms its circumferential end, for instance, in the shape of a sharp peak, and the die 5 forms a flat outside circumferential face. According to this invention, the reducing work of the pipe end diameter can be performed by small working force and in a short time, as per said execution example, and also the diameter reduction mainly consists of bending, and scarcely causes deformation. Accordingly, the production efficiency is improved and also a noise is small.