A low-alloy high-strength seamless steel pipe for oil country tubular goods, the steel pipe having a yield strength of 862 MPa or more, and having a composition that comprises, in mass%, C: 0.25 to 0.50%, Si: 0.01 to 0.40%, Mn: 0.3 to 1.5%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, AI: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0.5 to 0.8%, Mo: 0.5 to 1.3%, Nb: 0.005 to 0.05%, B: 0.0005 to 0.0040%, Ca: 0.0010 to 0.0020%, Mg: 0.001% or less, and N: 0.005% or less, optionally one or more selected from V: 0.02 to 0.3%, W: 0.03 to 0.2%, Ta: 0.03 to 0.3%, Ti: 0.003 to 0.10%, and Zr: 0.003 to 0.10%, and in which the balance is Fe and incidental impurities, the steel pipe having a microstructure in which the number of oxide-base nonmetallic inclusions including CaO, Al 2 O 3 , and MgO and having a major diameter of 5 μm or more in the steel, measured with a method as of description, and satisfying the composition ratios represented by the following formulae and is 10 or less per 100 mm 2 , and in which the number of oxide-base nonmetallic inclusions including CaO, Al 2 O 3 , and MgO and having a major diameter of 5 μm or more in the steel, and satisfying the composition ratios represented by the following formulae and is 30 or less per 100 mm 2 , CaO / Al 2 O 3 ≤ 0.25 1.0 ≤ Al 2 O 3 / MgO ≤ 9.0 CaO / Al 2 O 3 ≥ 2.33 CaO / MgO ≥ 1.0 wherein (CaO), (Al 2 O 3 ), and (MgO) represent the contents of CaO, Al 2 O 3 , and MgO, respectively, in the oxide-base nonmetallic inclusions in the steel, in mass%. the steel pipe having a yield strength of 862 MPa or more, and having a composition that comprises, in mass%, C: 0.25 to 0.50%, Si: 0.01 to 0.40%, Mn: 0.3 to 1.5%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, AI: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0.5 to 0.8%, Mo: 0.5 to 1.3%, Nb: 0.005 to 0.05%, B: 0.0005 to 0.0040%, Ca: 0.0010 to 0.0020%, Mg: 0.001% or less, and N: 0.005% or less, optionally one or more selected from V: 0.02 to 0.3%, W: 0.03 to 0.2%, Ta: 0.03 to 0.3%, Ti: 0.003 to 0.10%, and Zr: 0.003 to 0.10%, and in which the balance is Fe and incidental impurities, C: 0.25 to 0.50%, Si: 0.01 to 0.40%, Mn: 0.3 to 1.5%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, AI: 0.015 to 0.080%, Cu: 0.02 to 0.09%, Cr: 0.5 to 0.8%, Mo: 0.5 to 1.3%, Nb: 0.005 to 0.05%, B: 0.0005 to 0.0040%, Ca: 0.0010 to 0.0020%, Mg: 0.001% or less, and N: 0.005% or less, optionally one or more selected from V: 0.02 to 0.3%, W: 0.03 to 0.2%, Ta: 0.03 to 0.3%, Ti: 0.003 to 0.10%, and Zr: 0.003 to 0.10%, and in which the balance is Fe and incidental impurities, the steel pipe having a microstructure in which the number of oxide-base nonmetallic inclusions including CaO, Al 2 O 3 , and MgO and having a major diameter of 5 μm or more in the steel, measured with a method as of description, and satisfying the composition ratios represented by the following formulae and is 10 or less per 100 mm 2 , and in which the number of oxide-base nonmetallic inclusions including CaO, Al 2 O 3 , and MgO and having a major diameter of 5 μm or more in the steel, and satisfying the composition ratios represented by the following formulae and is 30 or less per 100 mm 2 , CaO / Al 2 O 3 ≤ 0.25 1.0 ≤ Al 2 O 3 / MgO ≤ 9.0 CaO / Al 2 O 3 ≥ 2.33 CaO / MgO ≥ 1.0 wherein (CaO), (Al 2 O 3 ), and (MgO) represent the contents of CaO, Al 2 O 3 , and MgO, respectively, in the oxide-base nonmetallic inclusions in the steel, in mass%. <EOS>