PURPOSE: To obtain the coil of rectangular cross section with good flatness by forming a coil of circular or elliptical cross section, after then, housing it in the rectangular groove of lower press and pressing it down with the upper pressing die having the projecting part of rectangular cross section.

CONSTITUTION: The required part in the longitudinal direction of the circular pipe of required length is bent alternately to the opposite direction with the bending device, and the coil of circular cross section having the prescribed bent shape is formed. Next the lower pressing die 18 having the rectangular groove 19 formed in correspondence with the bent shape of the coil of circular cross section by combinating the corner groove block 25 and the straight groove block 26 and the upper pressing die 21 having the projecting part 20 of rectangular cross section formed in correspondence with the rectangular groove 19 of the lower pressing die 18 by combining the corner part pressing down block 30 and the straight part pressing down block 31 are prepared. The coil of circular crossing section is housed in the rectangular groove 19 of the lower pressing die 18, and by pressing it down with the projecting part 20 of dies 21 of the upper pressing die 21, it is worked to the coil of rectangular cross section with good flatness.