To provide a method for preventing the temperature drop of a cast slab corner part in order to prevent any hot-rolling flaw generated when performing the blooming mill of the cast slab having square cross section cast through the continuous casting of high-alloy steel.

When performing the blooming mill of a cast slab 1 having square cross section cast through the continuous casting, all of four corner parts in the longitudinal direction of the cast slab are chamfered and removed by a grinder before the cast slab is heated to form chamfered portions 4 of 20C to 50C. Thereby, the temperature drop of the corner parts 2 in the longitudinal direction is prevented, and generation of flaws during the blooming mill of the cast slab 1 of high-alloy steel is prevented.