PURPOSE: To improve a contact between fuel and air and prevent heat being accumulated in a furnace itself by a method wherein a furnace floor of a solid fuel combustion furnace is made as a porous plate having several voids communicating to each other over an entire length of the floor.

CONSTITUTION: A furnace floor 2 arranged to be inclined within a main body 1 of the furnace made of refractory material is made as a porous plate made of metal such as nickel-chromium and refractory material such as alumina and silica and the like. A fuel supplying part 3 is formed by a fuel feeding hopper 31 and a fuel supplying screw type feeder 32. An air box is arranged below the furnace floor and air is supplied from a forced blowing fan 52 through a wind tunnel 53. Since combustion air is fed from a substantial entire surface of the furnace floor 2 into fuel 7, a better contact is found between the fuel and air. Air shows a low flow speed, resulting in that the fuel is not blown excessively and a thermal accumulation in the furnace floor itself can be prevented.