To improve ventilation hindrance at a furnace bed, ensure good combustion and realize mechanical ash discharge by forming a space at a lower portion of a combustion furnace to dispose therein a fixed fire grate, disposing a movable fire grate above the fixed fire grate and covering the movable fire grate with pebbles.

A combustion furnace comprises a combustion furnace main body 1, an ash collector 2 formed at a lower portion of the main body 1, a fixed fire grate 3 fixed to the main body 1 so as to define the ash collector 2, a movable fire grate 4 which is arranged on the fixed fire grate 3 in a sliding manner, a number of pebbles 5 covering the movable fire grate 4 so as to form a layer of a given thickness and a sliding device 6 for driving the movable fire grate 4. The main body 1 consists of a vertically cylindrical body which is substantially airtight, in which the space above the pebble layer is formed to be a combustion chamber 7 and a combustion gas exit 8 is formed at an upper portion of the combustion chamber 7.