An electric resistance welded steel pipe, wherein the electric resistance welded steel pipe comprises a welded portion comprising a heat-affected zone having a steel microstructure principally including a bainitic ferrite phase and/or a bainite phase, the steel microstructure at half the wall thickness of the heat-affected zone of the welded portion includes a bainitic ferrite phase and/or a bainite phase in a total area ratio of 90% or more relative to the entire microstructure at half the wall thickness of the heat-affected zone of the welded portion, in the steel microstructure located 1 mm in the wall thickness direction below the outer surface of the heat-affected zone of the welded portion, the bainitic ferrite phase and/or the bainite phase has an average grain size of 20 μm or less, and the average grain size of the bainitic ferrite phase and/or the bainite phase located 1 mm in the wall thickness direction below the outer surface of the heat-affected zone of the welded portion is 0.5 times or more and 2 times or less the average grain size of the bainitic ferrite phase and/or the bainite phase at half the wall thickness of the heat-affected zone of the welded portion, wherein the absorbed energy in a Charpy impact test at 0°C is 100 J or more as measured with respect to a region extending from a location 1 mm in the wall thickness direction below the outer surface of the welded portion to a location 11 mm in the wall thickness direction below the outer surface of the welded portion, and wherein the chemical composition of a base steel portion comprises, on mass% basis, C: 0.02 to 0.10%, Si: 0.05 to 0.30%, Mn: 0.80 to 2.00%, P: 0.030% or less, S: 0.0050% or less, Nb: 0.010 to 0.100%, Ti: 0.001 to 0.025%, and Al: 0.01 to 0.08%, optionally one, or two or more selected from, on mass% basis, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.10% or less, and Ca: 0.0050% or less, and the balance being Fe and incidental impurities, including O: 0.0030% or less and N: 0.0050% or less. the electric resistance welded steel pipe comprises a welded portion comprising a heat-affected zone having a steel microstructure principally including a bainitic ferrite phase and/or a bainite phase, the steel microstructure at half the wall thickness of the heat-affected zone of the welded portion includes a bainitic ferrite phase and/or a bainite phase in a total area ratio of 90% or more relative to the entire microstructure at half the wall thickness of the heat-affected zone of the welded portion, in the steel microstructure located 1 mm in the wall thickness direction below the outer surface of the heat-affected zone of the welded portion, the bainitic ferrite phase and/or the bainite phase has an average grain size of 20 μm or less, and the average grain size of the bainitic ferrite phase and/or the bainite phase located 1 mm in the wall thickness direction below the outer surface of the heat-affected zone of the welded portion is 0.5 times or more and 2 times or less the average grain size of the bainitic ferrite phase and/or the bainite phase at half the wall thickness of the heat-affected zone of the welded portion, wherein the absorbed energy in a Charpy impact test at 0°C is 100 J or more as measured with respect to a region extending from a location 1 mm in the wall thickness direction below the outer surface of the welded portion to a location 11 mm in the wall thickness direction below the outer surface of the welded portion, and wherein the chemical composition of a base steel portion comprises, on mass% basis, C: 0.02 to 0.10%, Si: 0.05 to 0.30%, Mn: 0.80 to 2.00%, P: 0.030% or less, S: 0.0050% or less, Nb: 0.010 to 0.100%, Ti: 0.001 to 0.025%, and Al: 0.01 to 0.08%, optionally one, or two or more selected from, on mass% basis, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, V: 0.10% or less, and Ca: 0.0050% or less, and the balance being Fe and incidental impurities, including O: 0.0030% or less and N: 0.0050% or less.