PURPOSE: To control an obstacle of slagging in burning by injecting slagging depressant and cooling medium into the high temp. part of a boiler burning petroleum coke.

CONSTITUTION: Slagging depressant which has one kind or more of compounds of Ca, Mg, Al, Si, Fe, etc. as effective components and is mixed in fuel with 0.5W0.01wt% desirable to the used quantity of fuel, is injected into a boiler or injected into the burning atmosphere of the boiler from another inlet different from a fuel inlet and also cooling medium composing of water or air is added to the depressant with a range of 1.5W0.5vol% desirable. Fuel is made harmless by forming high melting point material by the aid the slagging depressant. Fusion power of material which is not sufficiently changed into high melting point material is weakened by the injection of the cooling medium and the material is unable to adhere easily to the inlet of screen and a secondary superheater etc. and slagging in the burning time is controlled.