A high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing, the steel pipe having a composition containing, on a mass percent basis: C: 0.01% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.0% to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001% to 0.10%, N: 0.006% or less, Nb: 0.010% to 0.100%, and Ti: 0.001% to 0.050%, and optionally one or more of V: 0.1% or less, Mo: 0.5% or less, Cr: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, B: 0.0030% or less, Ca: 0.0005% to 0.0050% and REM: 0.0005% to 0.0050%, the remainder being Fe and incidental impurities, the steel pipe having a structure composed of 90% or more by volume of a bainitic ferrite phase as a main phase and 10% or less including 0% by volume of a second phase, the bainitic ferrite phase having an average grain size of 10 μm or less, wherein the average grain size is defined as the arithmetic mean of the equivalent circular diameter of the grains, the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion, a ratio in % of the fine Nb precipitates to the total amount of Nb being more than 75% on a Nb equivalent basis, wherein the amount of fine Nb precipitates is determined by electrolyzing an electroextraction test piece taken from a base material portion of an electric-resistance-welded steel pipe in an electrolyte solution of 10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol solution, filtering the resulting electrolytic residue through a filter having a pore size of 0.02 μm, and analyzing the amount of Nb passing through the filter, and a circularity of an end portion of the steel pipe defined by the following formula being 0.6% or less. Circularity % = maximum outer diameter mm φ of steel pipe − minimum outer diameter mm φ of steel pipe / nominal outer diameter mm φ × 100 C: 0.01% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.0% to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001% to 0.10%, N: 0.006% or less, Nb: 0.010% to 0.100%, and Ti: 0.001% to 0.050%, and optionally one or more of V: 0.1% or less, Mo: 0.5% or less, Cr: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, B: 0.0030% or less, Ca: 0.0005% to 0.0050% and REM: 0.0005% to 0.0050%, the remainder being Fe and incidental impurities, the steel pipe having a structure composed of 90% or more by volume of a bainitic ferrite phase as a main phase and 10% or less including 0% by volume of a second phase, the bainitic ferrite phase having an average grain size of 10 μm or less, wherein the average grain size is defined as the arithmetic mean of the equivalent circular diameter of the grains, the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion, a ratio in % of the fine Nb precipitates to the total amount of Nb being more than 75% on a Nb equivalent basis, wherein the amount of fine Nb precipitates is determined by electrolyzing an electroextraction test piece taken from a base material portion of an electric-resistance-welded steel pipe in an electrolyte solution of 10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol solution, filtering the resulting electrolytic residue through a filter having a pore size of 0.02 μm, and analyzing the amount of Nb passing through the filter, and a circularity of an end portion of the steel pipe defined by the following formula being 0.6% or less. Circularity % = maximum outer diameter mm φ of steel pipe − minimum outer diameter mm φ of steel pipe / nominal outer diameter mm φ × 100