A high strength seamless steel pipe for machine structure use superior in toughness and weldability characterized by containing, by mass%, C: 0.03 to less than 0.1%, Mn: 0.8 to 2.5%, Ti: 0.005 to 0.035%, Nb: 0.003 to 0.04%, and B: 0.0003 to 0.003%, limiting Si: 0.5% or less, Al: 0.05% or less, P: 0.015% or less, S: 0.008% or less, and N: 0.008% or less, further containing one or more of Ni: 0.1 to 1.5%, Cr: 0.1 to 1.5%, Cu: 0.1 to 1.0%, and Mo: 0.05 to 0.5%, and having a balance of Fe and unavoidable impurities, the metallurgical structure being a single phase structure of self-tempered martensite or a mixed phase structure of self-tempered martensite and lower bainite.