A thick steel plate for structural pipes or tubes, comprising: a chemical composition that contains, in mass%, C: 0.030 % to 0.100 %, Si: 0.01 % to 0.50 %, Mn: 1.50 % to 2.50 %, Al: 0.010 % to 0.080 %, Mo: 0.05 % to 0.50 %, Ti: 0.005 % to 0.025 %, Nb: 0.005 % to 0.080 %, N: 0.001 % to 0.010 %, O: 0.0005 % to 0.0050 %, P: 0.001 % to 0.010 %, S: 0.0001 % to 0.0010 %, optionally V: 0.005 % to 0.100 %, optionally one or more selected from the group consisting of Cu: 0.50 % or less, Ni: 0.50 % or less, Cr: 0.50 % or less, Ca: 0.0005 % to 0.0035 %, REM: 0.0005 % to 0.0100 %, and B: 0.0020 % or less, and the balance consisting of Fe and inevitable impurities, with the chemical composition having a carbon equivalent C eq as defined by the following Expression of 0.42 or more: C eq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5 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; and a microstructure at a mid-thickness part of the thick steel plate that is a dual-phase microstructure of ferrite and bainite with an area fraction of the ferrite being less than 50 %, and that contains ferrite grains with a grain size of 15 μm or less in an area fraction of 80 % or more with respect to the whole area of the ferrite, wherein total area fraction of microstructural constituents other than ferrite and bainite in the microstructure is up to 10 %, the plate thickness of the thick steel plate is 38 mm or more, and the steel plate satisfies a set of conditions including: a tensile strength being 620 MPa or more; and a Charpy absorption energy vE -20 °C at -20 °C at the mid-thickness part of 100 J or more. a chemical composition that contains, in mass%, C: 0.030 % to 0.100 %, Si: 0.01 % to 0.50 %, Mn: 1.50 % to 2.50 %, Al: 0.010 % to 0.080 %, Mo: 0.05 % to 0.50 %, Ti: 0.005 % to 0.025 %, Nb: 0.005 % to 0.080 %, N: 0.001 % to 0.010 %, O: 0.0005 % to 0.0050 %, P: 0.001 % to 0.010 %, S: 0.0001 % to 0.0010 %, optionally V: 0.005 % to 0.100 %, optionally one or more selected from the group consisting of Cu: 0.50 % or less, Ni: 0.50 % or less, Cr: 0.50 % or less, Ca: 0.0005 % to 0.0035 %, REM: 0.0005 % to 0.0100 %, and B: 0.0020 % or less, and the balance consisting of Fe and inevitable impurities, with the chemical composition having a carbon equivalent C eq as defined by the following Expression of 0.42 or more: C eq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5 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; and C: 0.030 % to 0.100 %, Si: 0.01 % to 0.50 %, Mn: 1.50 % to 2.50 %, Al: 0.010 % to 0.080 %, Mo: 0.05 % to 0.50 %, Ti: 0.005 % to 0.025 %, Nb: 0.005 % to 0.080 %, N: 0.001 % to 0.010 %, O: 0.0005 % to 0.0050 %, P: 0.001 % to 0.010 %, S: 0.0001 % to 0.0010 %, optionally V: 0.005 % to 0.100 %, optionally one or more selected from the group consisting of Cu: 0.50 % or less, Ni: 0.50 % or less, Cr: 0.50 % or less, Ca: 0.0005 % to 0.0035 %, REM: 0.0005 % to 0.0100 %, and B: 0.0020 % or less, and Cu: 0.50 % or less, Ni: 0.50 % or less, Cr: 0.50 % or less, Ca: 0.0005 % to 0.0035 %, REM: 0.0005 % to 0.0100 %, and B: 0.0020 % or less, and the balance consisting of Fe and inevitable impurities, with the chemical composition having a carbon equivalent C eq as defined by the following Expression of 0.42 or more: C eq = C + Mn / 6 + Cu + Ni / 15 + Cr + Mo + V / 5 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; and a microstructure at a mid-thickness part of the thick steel plate that is a dual-phase microstructure of ferrite and bainite with an area fraction of the ferrite being less than 50 %, and that contains ferrite grains with a grain size of 15 μm or less in an area fraction of 80 % or more with respect to the whole area of the ferrite, wherein total area fraction of microstructural constituents other than ferrite and bainite in the microstructure is up to 10 %, the plate thickness of the thick steel plate is 38 mm or more, and the steel plate satisfies a set of conditions including: a tensile strength being 620 MPa or more; and a Charpy absorption energy vE -20 °C at -20 °C at the mid-thickness part of 100 J or more. a tensile strength being 620 MPa or more; and a Charpy absorption energy vE -20 °C at -20 °C at the mid-thickness part of 100 J or more.