A high-strength seamless steel pipe for oil country tubular goods of a composition comprising C: 0.20 to 0.50 mass%, Si: 0.05 to 0.40 mass%, Mn: 0.1 to 1.5 mass%, P: 0.015 mass% or less, S: 0.005 mass% or less, Al: 0.005 to 0.1 mass%, N: 0.006 mass% or less, Cr: 0.1 to 2.5 mass%, Mo: 0.1 to 1.0 mass%, V: 0.03 to 0.3 mass%, Nb: 0.001 to 0.030 mass%, B: 0.0003 to 0.0030 mass%, O (oxygen): 0.0030 mass% or less, Ti: 0.003 to 0.025 mass%, optionally at least one selected from Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, W: 3.0 mass% or less, and Ca: 0.0005 to 0.0050 mass%, and the balance Fe and unavoidable impurities, and satisfying Ti/N = 2.0 to 5.5, whereas Ti and N represent the contents in mass% of respective elements Ti and N, wherein the high-strength seamless steel pipe has a structure in which a volume fraction of tempered martensite is 95% or more, and a prior austenite grain size number is 8.5 or more wherein the volume fraction of tempered martensite and the prior austenite grain size are measured utilizing the methods described in the description, and that contains nitride inclusions which have a size of 4 μm or more and whose number is 100 or less per 100 mm 2, nitride inclusions which have a size of less than 4 μm and whose number is 700 or less per 100 mm 2, oxide inclusions which have a size of 4 μm or more and whose number is 60 or less per 100 mm 2, and oxide inclusions which have a size of less than 4 μm and whose number is 500 or less per 100 mm 2, in a cross section perpendicular to a rolling direction, wherein the size and number of the nitride inclusions and the oxide inclusions are measured utilizing the methods described in the description, wherein the high-strength seamless steel pipe has a yield strength YS of 862 MPa or more measured utilizing the method described in the description, and wherein the nitride inclusions contain Ti and Nb as main components, and the oxide inclusions contain Al, Ca and Mg as the main components, whereas main component refers to when the elements are 65 mass% or more in total. wherein the high-strength seamless steel pipe has a structure in which a volume fraction of tempered martensite is 95% or more, and a prior austenite grain size number is 8.5 or more wherein the volume fraction of tempered martensite and the prior austenite grain size are measured utilizing the methods described in the description, and that contains nitride inclusions which have a size of 4 μm or more and whose number is 100 or less per 100 mm 2, nitride inclusions which have a size of less than 4 μm and whose number is 700 or less per 100 mm 2, oxide inclusions which have a size of 4 μm or more and whose number is 60 or less per 100 mm 2, and oxide inclusions which have a size of less than 4 μm and whose number is 500 or less per 100 mm 2, in a cross section perpendicular to a rolling direction, wherein the size and number of the nitride inclusions and the oxide inclusions are measured utilizing the methods described in the description, wherein the high-strength seamless steel pipe has a yield strength YS of 862 MPa or more measured utilizing the method described in the description, and wherein the nitride inclusions contain Ti and Nb as main components, and the oxide inclusions contain Al, Ca and Mg as the main components, whereas main component refers to when the elements are 65 mass% or more in total.