Excellent welding structural steel in low temperature toughness of the heat affected zone

The present invention provides a high strength thick steel plate for marine structures superior in weldability and lo characterized by casting molten steel containing, by mass%, C: 0.03 to 0.12%, Si: 0.05 to 0.30%, Mn: 1.2 to 3.0%, P: 0.015% or less, S: 0.001 to 0.015%, Cu+N