A steel plate for structural pipes or tubes, comprising: a chemical composition that contains, in mass%, C: 0.050 % to 0.080 %, Si: 0.01 % to 0.50 %, Mn: 1.50 % to 2.50 %, Al: 0.010% to 0.080 %, Cr: 0.05% to 0.50 %, Mo: 0.10% to 0.50 %, Ti: 0.005 % to 0.025 %, Nb: 0.005 % to 0.050 %, N: 0.001 % to 0.010 %, O: 0.0050 % or less, P: 0.010 % or less, S: 0.0020 % or less, and optionally one or more of V: 0.030 % or less, Cu: 0.50 % or less, Ni: 0.50 % or less, and Ca: 0.0005 % to 0.0035 %, the balance consisting of Fe and inevitable impurities, with the chemical composition satisfying a set of conditions including: a carbon equivalent C eq as defined by the following Expression of 0.43 or more, P cm as defined by the following Expression of 0.20 or less, and X as defined by the following Expression of 1.0 or more: C eq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5 P cm = C + Si / 30 + Mn / 20 + Cu / 20 + Mo / 15 + V / 10 + 5 ⁢ B X = 0.23 Cr + 0.125 Mo + 0.13 Nb + 0.24 V + 0.25 Ti / C and 0.01 < Y < 0.05 and Z < 3.10, where Y and Z are defined by: Y = A ∗ 55.85 Z = C / 12 − A ∗ A ∗ 1000000 A = Ti / 47.9 + Nb / 92.9 + V / 50.9 where each element symbol indicates content in mass% of the element in the steel plate and has a value of 0 if the element is not contained in the steel plate, wherein the steel plate has mechanical properties including: (a) a 0.5 % yield strength of 555 MPa or more; (b) a tensile strength of 625 MPa or more; and (c) a Charpy absorption energy vE -10°C at -10 °C at its mid-thickness part of 250 J or more, and the steel plate maintains the mechanical properties (a) to (c) even after subjection to heat treatment at 650 °C for 2 hours. C: 0.050 % to 0.080 %, Si: 0.01 % to 0.50 %, Mn: 1.50 % to 2.50 %, Al: 0.010% to 0.080 %, Cr: 0.05% to 0.50 %, Mo: 0.10% to 0.50 %, Ti: 0.005 % to 0.025 %, Nb: 0.005 % to 0.050 %, N: 0.001 % to 0.010 %, O: 0.0050 % or less, P: 0.010 % or less, S: 0.0020 % or less, and optionally one or more of V: 0.030 % or less, Cu: 0.50 % or less, Ni: 0.50 % or less, and Ca: 0.0005 % to 0.0035 %, P cm as defined by the following Expression of 0.20 or less, and X as defined by the following Expression of 1.0 or more: C eq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5 P cm = C + Si / 30 + Mn / 20 + Cu / 20 + Mo / 15 + V / 10 + 5 ⁢ B X = 0.23 Cr + 0.125 Mo + 0.13 Nb + 0.24 V + 0.25 Ti / C and 0.01 < Y < 0.05 and Z < 3.10, where Y and Z are defined by: Y = A ∗ 55.85 Z = C / 12 − A ∗ A ∗ 1000000 A = Ti / 47.9 + Nb / 92.9 + V / 50.9 where each element symbol indicates content in mass% of the element in the steel plate and has a value of 0 if the element is not contained in the steel plate, wherein the steel plate has mechanical properties including: (a) a 0.5 % yield strength of 555 MPa or more; (b) a tensile strength of 625 MPa or more; and (c) a Charpy absorption energy vE -10°C at -10 °C at its mid-thickness part of 250 J or more, and the steel plate maintains the mechanical properties (a) to (c) even after subjection to heat treatment at 650 °C for 2 hours. (a) a 0.5 % yield strength of 555 MPa or more; (b) a tensile strength of 625 MPa or more; and (c) a Charpy absorption energy vE -10°C at -10 °C at its mid-thickness part of 250 J or more, and the steel plate maintains the mechanical properties (a) to (c) even after subjection to heat treatment at 650 °C for 2 hours. <EOS>