The lubricating base oil of the invention satisfies at least one of conditions (a) or (b) below. The lubricating oil composition for an internal combustion engine according to the invention comprises the lubricating base oil of the invention, an ashless antioxidant containing essentially no sulfur as a constituent element, and at least one compound selected from among ashless antioxidants containing sulfur as a constituent element and organic molybdenum compounds. Also, a lubricating oil composition for a power train device according to the invention comprises the lubricating base oil of the invention, a poly(meth)acrylate-based viscosity index improver and a phosphorus-containing compound. (a) The saturated component content is 90% by mass or greater, and the proportion of cyclic saturated components among the saturated components is 10-40% by mass. (b) The condition represented by the following formula (1) is satisfied. 1.440≦n20−0.002×kv100≦1.453  (1) [wherein n20 represents the 20° C. refractive index of the lubricating base oil, and kv100 represents the kinematic viscosity at 100° C. (mm2/s) of the lubricating base oil.]