To secure sound shape/quality of band steel by preventing generation of weld zone damage (porosity DA, crack DC, etc.) that is prone to be caused at separation of a tab plate (melt-sticking to a butt welding zone) after welding, in a method of forming a butt joint for welding the end edges of a band steel by using a tab plate.

Succeedingly to a butt welding execution using a tab plate, the butt weld zone (W) is temporarily annealed with the tab plate melt-sticking, and then, the tab plate is separated from the band steel by bending the tab plate while the melt-sticking part of the tab plate is held in a soft state. In the temporary annealing performed with the tab plate melt-sticking, irregularities are caused in the temperature distribution in the width direction of the butt welding zone. Accordingly, after the tab plate separation, a heat treatment (regular annealing) is newly carried out to impart homogenous annealing texture to the entirety of the butt welding zone. The temporary annealing and the regular annealing processing can be efficiently performed by using a resistance heating apparatus that energizes by clamping the band steel (1) between a conductive table (31) (31) and an electrode (32) (32) that is controllably driven vertically.