A main purpose of the present invention is to provide a continuous casting method for a slab satisfying reductions of center segregation and center porosity, and inhibitions of surface cracks and internal cracks of a slab. The method includes a step of carrying out reduction on a slab having an unsolidified part by horizontal rolls and a step of alternatively carrying out reduction on the slab completely solidified by horizontal rolls and vertical rolls, wherein in the former step, reduction ratio of the slab is more than 0.5% and no more than 3% and a ratio of a width of the unsolidified part at the cross section of the slab to a width of a contact part of the slab and the rolls is 0 to 7.15, and in the latter step, each reduction ratio of the slab by the rolls is 5.4% to 6.8%.