To obtain a lubricating oil composition which is excellent in both frictional characteristics at a sliding part and water separation and makes it possible to reuse a water-soluble coolant for cutting and grinding by blending a lube base oil with a sulfur-containing extreme-pressure additive, a phosphoric ester or an amine salt thereof, and a polyalkylene glycol derivative.

The lube base oil used comprises one having a kinematic viscosity (40°C) of 10-500mm2/s, although the type used is not limited. The sulfur- containing extreme-pressure additive used may be one having sulfur atom(s) in the molecule and capable of being dissolved or uniformly dispersed in the base oil to exhibit extreme-pressure characteristics and excellent frictional characteristics, without limitation in particular. For example, an olefin sulfide is used in an amount of 0.1-10wt.% as the extreme-pressure additive. An example of phosphoric ester used is triaryl phosphate. An example of amine used for the amine salt is butylamine. The amine salt is blended in an amount of 0.001-10wt.%. An example of polyalkylene glycol derivative used is a polyoxy(ethylene propylene) glycol, which is blended in an amount of 0.00001-1wt.%.