An as-rolled electric resistance welded steel pipe for a line pipe, the steel pipe comprising a base metal portion and an electric resistance welded portion, wherein a chemical composition of the base metal portion consists of, in terms of % by mass: from 0.030 to 0.120% of C, from 0.05 to 0.30% of Si, from 0.50 to 2.00% of Mn, from 0 to 0.030% of P, from 0 to 0.0100% of S, from 0.010 to 0.035% of Al, from 0.0010 to 0.0080% of N, from 0.010 to 0.080% of Nb, from 0.005 to 0.030% of Ti, from 0.001 to 0.20% of Ni, from 0.10 to 0.20% of Mo, from 0 to 0.010% of V, from 0 to 0.0030% of O, from 0 to 0.0050% of Ca, from 0 to 0.30% of Cr, from 0 to 0.30% of Cu, from 0 to 0.0050% of Mg, from 0 to 0.0100% of REM, and the balance being Fe and impurities, wherein: F1 defined by the following Formula is from 0.300 to 0.350, in a metallographic microstructure of a wall thickness direction central portion of the base metal portion, a polygonal ferrite fraction is from 60 to 90%, an average crystal grain diameter is 15 μm or less, and a coarse crystal grain ratio, which is an areal ratio of crystal grains having a crystal grain diameter of 20 μm or more, is 20% or less, and a yield ratio in a pipe axis direction is from 80 to 95%, F 1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 wherein, in Formula , each of C, Si, Mn, Ni, Cr, Mo, V, and Nb represents mass% of a corresponding element. from 0.030 to 0.120% of C, from 0.05 to 0.30% of Si, from 0.50 to 2.00% of Mn, from 0 to 0.030% of P, from 0 to 0.0100% of S, from 0.010 to 0.035% of Al, from 0.0010 to 0.0080% of N, from 0.010 to 0.080% of Nb, from 0.005 to 0.030% of Ti, from 0.001 to 0.20% of Ni, from 0.10 to 0.20% of Mo, from 0 to 0.010% of V, from 0 to 0.0030% of O, from 0 to 0.0050% of Ca, from 0 to 0.30% of Cr, from 0 to 0.30% of Cu, from 0 to 0.0050% of Mg, from 0 to 0.0100% of REM, and the balance being Fe and impurities, wherein: F1 defined by the following Formula is from 0.300 to 0.350, in a metallographic microstructure of a wall thickness direction central portion of the base metal portion, a polygonal ferrite fraction is from 60 to 90%, an average crystal grain diameter is 15 μm or less, and a coarse crystal grain ratio, which is an areal ratio of crystal grains having a crystal grain diameter of 20 μm or more, is 20% or less, and a yield ratio in a pipe axis direction is from 80 to 95%, F 1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 wherein, in Formula , each of C, Si, Mn, Ni, Cr, Mo, V, and Nb represents mass% of a corresponding element. F1 defined by the following Formula is from 0.300 to 0.350, in a metallographic microstructure of a wall thickness direction central portion of the base metal portion, a polygonal ferrite fraction is from 60 to 90%, an average crystal grain diameter is 15 μm or less, and a coarse crystal grain ratio, which is an areal ratio of crystal grains having a crystal grain diameter of 20 μm or more, is 20% or less, and a yield ratio in a pipe axis direction is from 80 to 95%, F 1 = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 + V / 3 + Nb / 3 wherein, in Formula , each of C, Si, Mn, Ni, Cr, Mo, V, and Nb represents mass% of a corresponding element. <EOS>