A thick welded steel pipe excellent in low temperature toughness having a thickness of 25 to 45 mm that is formed of a base material steel plate formed into a pipe shape being seam welded and has a coarse-grained HAZ, wherein the base material steel plate has a chemical composition containing, in mass%, C: 0.03% to 0.085%, Mn: 1.45% to 1.85%, Ti: 0.005 to 0.020%, Nb: 0.005 to 0.050%, and O: 0.0005 to 0.005%, and Si: limited to 0.15% or less, Al: limited to 0.015% or less, P: limited to 0.02% or less, S: limited to 0.005% or less, and Mo: limited to 0.20% or less, further optionally containing one type or two types or more of, in mass%, Cu: 0.70% or less, Ni: 0.70% or less, Cr: 1.00% or less, V: 0.10% or less, Zr: 0.050% or less, and Ta: 0.050% or less and/or one type or two types or more of, in mass%, Mg: 0.0100% or less, Ca: 0.0050% or less, and REM: 0.0050% or less and having the contents of Mn and Mo satisfying (Expression 1) below, a Pcm obtained by (Expression 2) below being 0.16 to 0.19, and a balance being composed of Fe and inevitable impurities, a metal structure of the base material steel plate consists of ferrite being 30 to 95% in an area ratio and a low temperature transformation structure, and in a metal structure of the coarse-grained HAZ, an area ratio of grain boundary ferrite is 1.5% or more, the total area ratio of the grain boundary ferrite and intragranular ferrite is not less than 11% nor more than 90%, an area ratio of MA is 10% or less, and its balance is composed of bainite 1.2325 ≦ 0.85 × Mn − Mo ≦ 1.5215 Pcm = C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + V / 10 In (Expression 2),, [Si], [Mn], [Cu], [Cr], [Ni], [Mo], and denote the contents of C, Si, Mn, Cu, Cr, Ni, Mo, and V [mass%] respectively. C: 0.03% to 0.085%, Mn: 1.45% to 1.85%, Ti: 0.005 to 0.020%, Nb: 0.005 to 0.050%, and O: 0.0005 to 0.005%, and Si: limited to 0.15% or less, Al: limited to 0.015% or less, P: limited to 0.02% or less, S: limited to 0.005% or less, and Mo: limited to 0.20% or less, further optionally containing one type or two types or more of, in mass%, Cu: 0.70% or less, Ni: 0.70% or less, Cr: 1.00% or less, V: 0.10% or less, Zr: 0.050% or less, and Ta: 0.050% or less and/or one type or two types or more of, in mass%, Mg: 0.0100% or less, Ca: 0.0050% or less, and REM: 0.0050% or less and having the contents of Mn and Mo satisfying (Expression 1) below, a Pcm obtained by (Expression 2) below being 0.16 to 0.19, and a balance being composed of Fe and inevitable impurities, a metal structure of the base material steel plate consists of ferrite being 30 to 95% in an area ratio and a low temperature transformation structure, and in a metal structure of the coarse-grained HAZ, an area ratio of grain boundary ferrite is 1.5% or more, the total area ratio of the grain boundary ferrite and intragranular ferrite is not less than 11% nor more than 90%, an area ratio of MA is 10% or less, and its balance is composed of bainite 1.2325 ≦ 0.85 × Mn − Mo ≦ 1.5215 Pcm = C + Si / 30 + Mn + Cu + Cr / 20 + Ni / 60 + Mo / 15 + V / 10 In (Expression 2),, [Si], [Mn], [Cu], [Cr], [Ni], [Mo], and denote the contents of C, Si, Mn, Cu, Cr, Ni, Mo, and V [mass%] respectively.