To detect a heavy oil content eluted into a cleaning oil easily and in a short period of time and accurately judge the time of ending a cleaning process based on the tendency of elution, in cleaning the oil system of petroleum refinery facilities.

In carrying out a circulatory cleaning process of the petroleum refinery facilities using a light oil as a cleaning oil, the heavy oil content eluted into the cleaning oil is determined by measuring the index of refraction of the cleaning oil over time, and the end of the cleaning process is judged based on the actual measurements.