A high-strength steel pipe comprising: a base metal, wherein the base metal comprises, by mass%: C: 0.02% to 0.09%, Mn: 0.4% to 2.5%, Cr: 0.1% to 1.0%, Ti: 0.005% to 0.03%, Nb: 0.005% to 0.3%, at least one of Ni: 0.65% or less, Cu: 1.5% or less, Mo: 0.3% or less, and V: 0.2% or less, and a balance consisting of Fe and inevitable impurities, and wherein, in the base metal, Si is limited to 0.6% or less, Al is limited to 0.1% or less, P is limited to 0.02% or less, S is limited to 0.005% or less, N is limited to 0.008% or less, a bainite transformation index BT obtained by the following equation is 650°C or lower, and a microstructure of the base metal is a single bainite microstructure including a first bainite and a second bainite, the first bainite being a gathered microstructure of a bainitic ferrite including no carbide, and the second bainite being a mixed microstructure of a bainitic ferrite including no carbide and cementite between the bainitic ferrites, BT = 830 − 270 C − 90 Mn − 37 Mo − 70 Ni − 83 Cr where the, [Mn], [Mo], [Ni], and [Cr] are the amounts of C, Mn, Mo, Ni, and Cr, respectively. a base metal, wherein the base metal comprises, by mass%: C: 0.02% to 0.09%, Mn: 0.4% to 2.5%, Cr: 0.1% to 1.0%, Ti: 0.005% to 0.03%, Nb: 0.005% to 0.3%, at least one of Ni: 0.65% or less, Cu: 1.5% or less, Mo: 0.3% or less, and V: 0.2% or less, and a balance consisting of Fe and inevitable impurities, and wherein, in the base metal, Si is limited to 0.6% or less, Al is limited to 0.1% or less, P is limited to 0.02% or less, S is limited to 0.005% or less, N is limited to 0.008% or less, a bainite transformation index BT obtained by the following equation is 650°C or lower, and a microstructure of the base metal is a single bainite microstructure including a first bainite and a second bainite, the first bainite being a gathered microstructure of a bainitic ferrite including no carbide, and the second bainite being a mixed microstructure of a bainitic ferrite including no carbide and cementite between the bainitic ferrites, BT = 830 − 270 C − 90 Mn − 37 Mo − 70 Ni − 83 Cr where the, [Mn], [Mo], [Ni], and [Cr] are the amounts of C, Mn, Mo, Ni, and Cr, respectively. C: 0.02% to 0.09%, Mn: 0.4% to 2.5%, Cr: 0.1% to 1.0%, Ti: 0.005% to 0.03%, Nb: 0.005% to 0.3%, at least one of Ni: 0.65% or less, Cu: 1.5% or less, Mo: 0.3% or less, and V: 0.2% or less, and Ni: 0.65% or less, Cu: 1.5% or less, Mo: 0.3% or less, and V: 0.2% or less, and a balance consisting of Fe and inevitable impurities, and wherein, in the base metal, Si is limited to 0.6% or less, Al is limited to 0.1% or less, P is limited to 0.02% or less, S is limited to 0.005% or less, N is limited to 0.008% or less, a bainite transformation index BT obtained by the following equation is 650°C or lower, and a microstructure of the base metal is a single bainite microstructure including a first bainite and a second bainite, the first bainite being a gathered microstructure of a bainitic ferrite including no carbide, and the second bainite being a mixed microstructure of a bainitic ferrite including no carbide and cementite between the bainitic ferrites, BT = 830 − 270 C − 90 Mn − 37 Mo − 70 Ni − 83 Cr where the, [Mn], [Mo], [Ni], and [Cr] are the amounts of C, Mn, Mo, Ni, and Cr, respectively.