PURPOSE: To prevent generation of various flaws by forming a side face of a rear end part of a flat part at a prescribed constant inclination angle, and forming an inclination angle of a side wall of a thin wall side of a deformed section strip, to a desired angle.

CONSTITUTION: As for a stock W, the center part of a part Wa to become a thin wall part by an inclined part 33 is recessed, and a groove G is brought to forming therein. This groove G is formed in a shape whose section is trapezoidal, in accordance with the inclined part 33, and its depth also becomes deeper gradually, and becomes equal to a wall thickness part H of a thick wall part and a thin wall part. The stock W is fed by one pitch and both side parts of the groove G are formed by a tip part 35 of a flat part 34. In this case, the upper face 35a is fitted into the lower end open part of the groove G. The tip part 35 of the flat part 34 is pushed into the groove G and both side faces of the groove G are formed. Both the side faces of the groove G are corrected to 90° being a desired inclination angle gradually whenever the stock is fed by one pitch. In this way, both the side faces of the groove G are brought to finish forming by the rear end part 36 of the flat part 34, and a deformed section strip A being a finished product is formed.