In response to recent demands for effective use of energy and increased awareness of environmental protection, MHI has developed a new CUF boiler capable of ensuring a low NO_x emission and burning coal stably at a low air ratio. In the CUF type boiler, its burner is installed at the four surrounding walls having high radiative intensity and the fuel and the air are injected slightly downwards to be fired while they are made to swirl over the entire furnace. Becauce this boiler type is excellent in fuel ignitability and can maintain a sufficient fuel residence time in its reduction zone, its development has always been oriented towards combustion of hard-to-burn coals such as anthracite. In the CUF boiler newly developed, those features have been adopted towards low NO_x and high efficiency. For further decrease of the NO_x level, optinum conditions of the burner location and the fuel injection direction were found out using a 1 / 15 scale cold model test, which made it possible to achieve coal firing at a lower NO_x and higher effeciency than ever. Based on these results, a CUF boiler of evaporation 350t / h was made and this boiler has successfully been operating. Combustion state in the furnace has been found very stable and operation of this boiler using a low air ratio of excess O₂ obout 1% is made available. Comparison with the conventional boiler of the same scale shows that there is the effect of NO_x decrease by about 30% on a basis of the same unburned carbon ratio in ash.