THICK STEEL PLATE

A thick steel plate, the chemical composition of which comprises, in mass%, C in the amount of 0.01-0.20%, Si in the amount of 0.10-0.25%, Mn in the amount of 1.30-2.50%, P in the amount of 0.01% or less, S in the amount of 0.0010-0.0100%, Ti in the amount of 0.005-0.030%, Al in the amount of 0.003% or less, O in the amount of 0.0010-0.0050%, N in the amount of 0.0100% or less, Cu in the amount of0-0.50%, Ni in the amount of 0-1.50%, Cr in the amount of 0-0.50%, Mo in the amount of 0-0.50%, V in the amount of 0-0.10%, and Nb in the amount of 0-0.05%, with the remainder constituting Fe and impurities, wherein the steel also contains a composite inclusion in which MnS is present around a Ti oxide. The surface area ratio of MnS in a cross-section of the composite inclusion is at least 10% and less than 90%, the proportion which MnS constitutes along the peripheral length of the composite inclusion is at least 10%, and the number density of composite inclusions that have a particle diameter of 0.5-5.0 [mu]m is 10