PURPOSE: To prevent the occurrenc of an unsound operating conditon, by providing a contaminating condition detecting means, an operating condition detecting means, a conputing means for the degree of priority of a contaminating condition, a computing means for the degree of priority of an operating condition and a deciding means for the degree of priority.

CONSTITUTION: A computer 104 for the degree of a contaminatng condition calculates the degree of priority of a contaminating condition from a signal from a contaminating condition computer 101 and with the aid of a setter 103 for the degree of priority of a contaminating condition. A computer 106 for the degree of priority of a boiler condition computes the degree of priority of a boiler condition by means of signal inputs from a boiler condition computer 102 and a setter 105 for the degree of priority of a boiler condition. A computer 108 for deciding a soot blower operating spot decides a soot blower operating spot by means of an efficiency signal from the boiler condition computer 102 and a signal from a computer 107 for the degree of priority of a composite condition. Based on the selective results, a soot blower 27, positioned facing a furnace water wall part 12, is operated to perform washing of a heating surface.