PURPOSE: To restrain the development of surface crack at the time of rolling a slab and to reduce the surface flaw on a product.

CONSTITUTION: In this slab, the thickness in the spans of 1/6 width from both ends in the width direction of the slab is thicker than the average thickness in the span near the center of the width direction from the above spans. Further, the average thickness He in the spans of 1/6 width from the both ends of the slab is made to be the thickness calculated in the following inequality by using the average thickness Hc in the span near the center of the width direction from the above spans. He=α(Hc-h)+h. Wherein, 1.1≤α≤1.4, h: slab thickness after hot rolling. Further, a mold, to which related to the gap between the inner walls of the cross section thereof crossing at the right angle in the casting direction, the gap between the inner walls of the mold corresponding to the slab thickness in the spans from the both ends in the width direction to the 1/6 width of the cast slab is larger than the average value of the gap between the inner walls in the thickness direction in the span near the center of the width from the above spans, is used.