PURPOSE: To prevent the development of center segregation and center porosity at the time of continuously casting a rectangular cast slab.

CONSTITUTION: A flatness ratio of the cast slab 1 is made to be ≥1.1. Rolling reduction terminals 4 are arranged at the position (y) satisfying the inequality I or the inequality II. Casting velocity is adjusted and the solid phase ratio at the center part in the cross section of the cast slab 1 at the inlet side of the rolling reduction terminals 4 is made to be 0.6-0.95. The rolling reduction terminal 4 having ≥200mm radius of curvature in the axial direction of the cast slab at the part executing the rolling reduction to the cast slab 1 is used, and the cast slab is drafted in 5-40% draft in the thickness direction of the cast slab 1 at 80-540mm/min rolling reduction speed. Then, the inquality I and the inquality II are expressed as follows, in the case of 0≤D<250, 0.02×D≤y (m) ≤0.06×D...(1) and in the case of 250≤D, 0.02×D≤y (m) ≤0.067×(D-250)+15...(II). Wherein, D: thickness of the cast slab (mm).