PURPOSE: To easily integrally work a welding end part and easily ensure the strength of the part by forming a door beam by an open pipe which has a slit of a specified width extending over the whole length thereof and contains specified percent carbon by weight, and integrally forming bracket parts formed by spreading the slit into a flat surface at both ends of the open pipe.

CONSTITUTION: An open pipe is formed by steep material containing 0.2-0.25wt.% carbon, and at that time, a slit width S is set to 1.0mm or more and high- strengthen working is conducted for the middle part 5 by high-frequency quenching. Though before and after heat treatment, the slit width S is changed linearly, if the slit width S before heat treatment is set to 1.0mm or more, there is no possibility that the edges of the slit come into contact with each other. The change of the slit width S caused by high-frequency quenching is little influenced by a difference in diameter and thickness. Subsequently, the open pipe is flared from the slit 2 to form flat bracket parts 3, 4 by flat forging. The bracket parts 3, 4 are welded on the door.