PURPOSE: To reduce production of an unburnt content and NOx, by a method wherein, when coal is brought to a water slurry state, viscosity is determined according to a ratio of fixed oxygen to a volatile content.

CONSTITUTION: A relation between slurry fuel viscosity cp and a fuel ratio FR (of fixed carbon to a volatile content) is as shown in diagram. Depending upon a kind of coal, burning characteristics of slurry fuel are more excellent (white) or more adverse (oblique line) than pulverized coal, but the burning characteristics are varied with viscosity cp of slurry. From the diagram, relation therebetween is set to μ≤1,510exp (-0.465FR), wherein μ is viscosity cp of coal-water slurry. In relation between the degree F of pulverization (200 mesh transmitting percent) and a fuel ratio FR, excellent burning characteristics are provided in the case of F≥15.5FR+41.2. Thus, coal water slurry fuel is fed as property, satisfying the above conditions, to a burner.