PROBLEM TO BE SOLVED: To provide a low alloy high strength seamless steel pipe for an oil well having excellent sulfide stress corrosion cracking resistance and its manufacturing method.SOLUTION: A seamless steel pipe contains a composition comprising, by mass%, C:0.15 to 0.50%, Si:0.1 to 1.0%, Mn:0.3 to 1.0%, P:0.015% or less, S:0.005% or less, Al:0.01 to 0.10%, N:0.01% or less, Cr:0.1 to 1.7%, Mo:0.4 to 1.1%, V:0.01 to 0.12%, Nb:0.01 to 0.08%, Ti:0.005 to 0.03% and B:0.0005 to 0.0030% and has each segregation degree of Mn, Mo and Cr in a segregation part of 1.5 or less. A manufacturing method of the seamless steel pipe includes applying, to the seamless steel pipe, segregation lowering treatment for heating the seamless steel pipe at T (°C) in range of 1100°C to 1300°C and maintaining for a definite period of time and then cooling it, and thereafter, applying hardening treatment one or more times and then tempering treatment.
