To provide a cutting oil agent at a low cost, which is obtained from mineral oil and which has various viscosity, lubricating property, cooling performance, flushing property and antibacterial performance, and which prevents environmental pollution.

The cutting oil agent is obtained by radiating an ultrasonic wave having a radiation frequency of 20 to 50 KHz and a radiation intensity of 1.0 to 5.0 W/cm2 to mineral oil having a required viscosity, so that the paraffin chains are cut by a high temperature and high pressure reaction field and flow speed of liquid caused by collapse of cavities and that the molecular weight and viscosity are reduced, then radiating an ultrasonic wave having a radiation frequency of 100 to 300 KHz and a radiation intensity of 20 to 120 W/cm2 to the resulting mineral oil, so that performances are stabilized by the formation of fine and dense air bubbles or droplets, and further, radiating an ultrasonic wave having a radiation frequency of 500 to 2,000 KHz and a radiation intensity of 20 to 120 W/cm2 to the resulting mineral oil, so that additional functions are imparted to the mineral oil.