PURPOSE: To provide a cooling method for a hot steel plate eliminating uneven cooling in a transverse direction.

CONSTITUTION: Before or after a high quenching nozzle 2 which is arranged orthogonally with the sending direction of a hot steel plate 4 an auxiliary quenching nozzle 5 which ejects cooling water only to the center part of the steel plate is arranged. The cooling water is ejected from the high quenching nozzle 2 and the auxiliary quenching nozzle to the hot steel plate 4 carried by a steel plate carrying roller 1. The cooling for the center part of the steel plate is promoted. The cooling speed difference between the center part and the edge part of the steel plate is eliminated and whole part is uniformly cooled over the transverse direction. The thin wide steel plate excellent in its flatness is manufactured by a roller quenching device.