PROBLEM TO BE SOLVED: To obtain a hydraulic oil composition which prevents generation of rust or sludge and deterioration of the hydraulic oil and permits stable operation of a hydraulic operating system even when it is used for the hydraulic operating system of high performances obtained by enhancing speed/precision of a servo valve, or the like, and to provide an operation method for the hydraulic operating system. SOLUTION: The hydraulic oil composition comprises at least one selected from among mineral oils, oil and fat and synthetic oils as a base oil, and incorporated therewith, (A) 0.01-2 mass% of an ashless rust-proofing agent having a total base value of 2-100 mgKOH/g, (B) 0.01-2 mass% of an ashless antioxidant, and (C) 0.005-5 mass% of an ashless wear-preventing agent, and is substantially free from zinc dithiophosphate. COPYRIGHT: (C)2005,JPO&NCIPI