PURPOSE: To reduce the development of longitudinal crack in a slab by supplying powder having the specific viscosity and crystallizing temp. in range of the specific distance from center of long side face of a mold for middle carbon steel slab.

CONSTITUTION: A submerged nozzle 1 having two holes is submerged into mol ten steel M in the mold 2 and the middle carbon steel slab containing 0.09W0.16 wt.% C is cast. Then, only in the range within 200 mm from the center of the long side of the mold 2 for the slab, the powder 4 having 1.1 poise at 1300°C and ≤1100°C crystallizing temp. is used and further the powder 4 having ≥1100°C crystallizing temp. is used to the part except near center part. By this method, even in case molten steel temp. near the center part of the long side face of the mold is low, slag-making is sufficiently executed and perfectly melted, and the longitudinal crack in the slab is prevented.