Seamless steel pipe and a method of manufacturing the same

Even thick, with high strength and toughness, provides a seamless steel pipe. Seamless steel pipe according to the present embodiment, by mass%, C: 0.03~0.08%, Si: 0.25% or less, Mn: 0.3~2.0%, P: 0.05% or less , S: 0.005% or less, Al: 0.001~0.10%, Cr: 0.02~1.0%, Ni: 0.02~1.0%, Mo: 0.02~0. 8%, N: 0.002~0.008%, Ca: 0.0005~0.005%, and, Nb: contains 0.01% to 0.1%, the balance being Fe and impurities, 50 mm has a wall thickness of at least the seam of the cross section perpendicular to the axial direction of steel pipe, the old austenite grains having an average grain in the surface layer is a region of 500 [mu] m × 500 [mu] m centered on the position of the surface of 2mm depth diameter of less than 80 [mu] m, and an average grain size of prior austenite grains in the surface layer, in the thickness of the cross section The difference between the average crystal grain size of the prior austenite grains in the thickness center portion is a region of 500 [mu] m × 500 [mu] m around the position is less than 50 [mu] m.