To prevent breaking in passing a plate, in the laser butt welding of a steel strip on a process line, by implementing heat input control, controlling the formation of defects in a joined part, especially the formation of coarse grains in a heat affected area on a beam irradiation side, and thereby enhancing the performance of a joint without a post heat treatment.

Welding is performed such that, in the butt welding conditions, a laser output P (kw) and welding speed v (m/min) are given to the following equation, with the calculated heat input HI controlled to be 1.6 to 5.0: HI=P/v. With a steel strip welded under this condition, the crystal grain density of a welding heat affected area can be made 10 pieces/μm2 or above, resulting in the improvement of the life against a bending action, and preventing breaking even if the steel strip is passed in the process line without applying a post heat treatment. Furthermore, this welding method can be applied to laser welding using a filler wire.