A high-strength steel sheet is provided which, even when subjected to long-term stress-relief annealing after welding, decreases little in strength and which has satisfactory low-temperature HAZ toughness. The high-strength steel sheet has a chemical composition adequately regulated and has a CP value defined by the following equation (1) of 5.40% or higher and a carbon equivalent (Ceq) defined by the following equation (2) of 0.45% or lower. CP value=125[Ti]+111[Nb]+60[V]+15[Mo] (1) ([Ti], [Nb], [V], and [Mo] indicate the contents (mass %) of Ti, Nb, V, and Mo, respectively.) Ceq=[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Cu]+[Ni])/15 (2) ([C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] indicate the contents (mass %) of C, Mn, Cr, Mo, V, Cu, and Ni, respectively.