Disclosed is a lubricant base oil composition that comprises (a) from 80 to 98% by mass of a naphthenic synthetic lubricant base oil having a flash point of not lower than 140 C. and (b) from 2 to 20% by mass of an alcohol/fatty acid ester having a flash point of not lower than 150 C. of which the alcohol has a gem-type dimethyl structure, or comprises (a) from 85 to 99% by mass of a naphthenic synthetic lubricant base oil having a flash point of not lower than 140 C. and (c) from 1 to 15% by mass of a mineral oil having a kinematic viscosity at 40 C. of from 0.9 to 9.6 mm²/sec, a boiling point under normal pressure of from 150 to 400 C. and a % CA measured through ring analysis (in n-d-M process) of at most 10%. The lubricant base oil composition has a flash point of not lower than 150 C. and has a high traction coefficient at high temperatures, and its low-temperature flowability characteristic is good. The oil composition is favorable for traction drive fluid.