To solve the problems that biodegradation requires high cost and long time and aeration requires to constantly check if cleaning is perfectly done or not and can hardly automated although biodegradation or aeration as various polluted soil cleaning methods against frequently happening cases of pollution exceeding a regulation value in the Soil Pollution Control Act enforced through the Ministry of the Environment is executed.

By applying the principle that a volatile organic compound in water cannot exist in high temperature water, sewage is heated to a high temperature in a heating tank to perfectly clean it. At a midpoint of discharge of its cleaned water, heat is radiated by a heat exchanger provided in the sewage of an upstream tank to drain it off outside as cold water. The sewage warmed by heat exchange in the upstream tank is filled into the empty heating tank until a regular water surface and heated again, thereby emitting an underwater compound to clean water. Then, it is discharged. By repeatedly carrying out this operation, cleaning of the sewage can be realized by continuous operation within a heat economy. The cleaning method by high temperature heating roughly perfectly cleans the sewage to clear the regulation value. Thus, discharge can be carried out without requiring inspection of whether cleaning is perfect or not one by one, thereby enabling automatic operation.