PURPOSE: To prevent an air nozzle for blowing up a sand layer from blocking and corrosion and improve the reliability of the titled furnace by a method wherein a granular solid is spread all over the floor of a combustion chamber for forming a granular solid layer in which an opening of a combustion air nozzle is buried.

CONSTITUTION: A fluidized layer type furnace blows out preheated air through a wind box 1 by an air duct 2 and an air nozzle 3, which fluidizes a sand layer 6 on a furnace floor 8 at the lower part of a combustion chamber 9 and burns a thing to be burnt, such as sludge within the heated fluidized sand layer 6 in the presence of a flame blown out from a burner (not shown in a sketch). In this case, a granular solid layer is formed by spreading pebbles all over a space between the furnace floor 8 and the sand layer 6, which is pressed down by a pebbles retainer 5, such as a wire net, and an opening of the above air nozzle 3 is buried in the granular solid layer. By means of the above, a fused substance of the thing to be burnt is prevented from reaching the air nozzle 3 and the nozzle 3 is prevented from blocking and corrosion.