PURPOSE: To enhance the oil oxidation ratio to the supply amount of gas and to promote the deterioration of oil by performing control so that the valve arranged on the way of the gas (oxygen or air) supply pipe to an oil container is opened and closed corresponding to the internal pressure of the oil container.

CONSTITUTION: A control part 2 has a gas supply control means controlling the opening and closing of a valve 16 on the basis of the detection value of a pressure sensor 17. When the oil (e.g., gasoline) 6 in a hermetically closed oil container 5 is oxidized by the gas in the container 5 and the pressure in the container 5 lowers to a lower limit set value, the valve 16 is opened to supply the gas into the container 5 from a supply source 14. Subsequently, when the pressure in the container 5 rises to an upper limit set value, the valve 16 is closed to stop the supply of the gas. When the oil is oxidized by the supplied gas and the pressure in the container 5 lowers to the lower limit set value, the above mentioned operation is repeated. Therefore, all of the gas supplied to the container 5 is consumed in the oxidation of the oil 6 and the oil 6 can be rapidly deteriorated. Since the oil 6 is not discharged to the atmosphere, no environmental pollution is generated.