PURPOSE: To obtain inexpensively a thick steel plate having no work contraction without performing special refining and steel making and without decreasing considerably [P] by subjecting the thick steel plate which is hot rolled, hardened and tempered to weak water cooling at a specific cooling rate to suppress residual stress.

CONSTITUTION: A thick steel plate consisting of, for example, 9% Ni steel is subjected to weak water cooling at 15°C/sec cooling rate after tempering in the stage of assuring low-temp. toughness thereof by performing hardening and tempering after hot rolling. The steel plate is otherwise weakly cooled at a cooling rate of 5W15°C/sec after tempering and is then subjected to leveling at ordinary curvature. The residual stress is suppressed by the above-mentioned methods. The residual stress occurring in water cooling after tempering is suppressed without deteriorating the low-temp. toughness simply by controlling the cooling rate after hardening and tempering according to the method of this invention. The thick steel plate having high toughness without having residual stress is thus inexpensively obtd. even with the kind of steel required to assure the low- temp. toughness such as 9% Ni steel, 3% Ni steel or the like.