Lubricant base oil and, as the composition total volume basis, 0.05 to 5% by weight of (R ¹ O) ₃ P = O phosphoric acid ester represented by [R ¹ is alkyl having from 6 to 15 aryl group and a saturated hydrocarbon group having 1 to 20 carbon atoms one and, R ¹ each other, each the same or different may be good; and 0.01 to 2% by weight of _{^{(R 2 O) n P (=}} O) (OH) (3-n) to show that acidic phosphate [R ² is an alkyl group having 4 to 20, n is 1 or 2, R ² may be the same each other or different], and containing 0.5 to 5% by weight of the sulfur compounds, the lubricating oil composition for machine tools.