PURPOSE: To perform high speed gas fusion cutting at the fusion cutting width equal to that at the low speed gas fusion cutting by arranging plural blow pipes in the slab cutting direction and assigning fusion cutting depth to each blow pipe.

CONSTITUTION: Plural blow pipes 2 are disposed in the fusion cutting direction A of a slab 1 such as bloom or slab and fusion cutting depth is assigned to each blow pipe 2, thence the slab 1 is fusion-cut. If the fusion cutting is made in this way, the upper part 1a of the slab 1 is fusion-cut by the blow pipes 2 on the leading side, following to which the lower part 1b is fusion-cut by the succeeding blow pipes 2. By sharing of the fusion cutting depth by the respective blow pipes 2, the slab 1 is cut at a high speed.