PURPOSE: To obtain advantageously a thick steel plate having excellent internal quality and high toughness by soaking a continuous casting billet right after cutting of an aluminum killed steel, rolling the same to a slab of a prescribed shape ratio and subjecting the slab to a dehydrogenation treatment then reheating the slab, rolling the heated slab to a finish thickness and cooling acceleratively the rolled plate.

CONSTITUTION: A continuous casting strand of an aluminum killed steel contg. ≤0.07wt% Al is hot charged in a decomposition soaking furnace in the state of the continuous casting billet right after cutting to a prescribed length. The continuous casting billet is rolled to a slab under strong rolling reduction in this state to press the porosity, that is, loose structure, at the center in the wall thickness of the billet and the possibility for delayed cracking of the thick steel plate occuring in hydrogen occlusion is eliminated by the succeeding dehydrogenation treatment. The slab subjected to the dehydrogenation treatment is reheated to deposit finely AlN whereby the thick plate rolling is accomplished. The rolled plate is subjected to accelerated cooling at ≥15°C/min heat extraction rate from the Ar3 or the temp. not lower by 40°C than said point down to between 500W350°C after the end of the thick plate rolling, whereby the intended thick steel plate having high toughness is obtd. The value of the shape ratio R (shown by the equation) in the above-described slab rolling is so set as to attain 0.5.