To provide a combustion furnace capable of completely burning a solid burned target mainly composed of an organic substance.

This combustion furnace 1 comprises a continuous supply unit 2, a lower preheating portion 3, a primary combustion portion 4 performing primary combustion, a secondary combustion portion 5 performing second combustion, and an exhaust pipe 6. The primary combustion portion 4 has a cylindrical circular primary air chamber 46 and a cylindrical primary combustion chamber 47, and has a plurality of tilted through-holes 42a formed in the circumferential direction at a lower portion of a first inner cylinder 42 partitioning the primary air chamber 46 and the primary combustion chamber 47. The secondary combustion portion 5 has a cylindrical circular secondary air chamber 56 and a cylindrical secondary combustion chamber 57, and comprises a plurality of tilted through-holes 52a formed in the circumferential direction at a lower portion of a second inner cylinder 52 partitioning a secondary air chamber 56 and a secondary combustion chamber 57. The swirling primary and secondary combustion air flowing downward in the primary and secondary air chambers 46, 56 are guided in the primary and secondary combustion chambers 47, 57 in the direction of tilt by the tilted through-holes 42a, 52a to form climbing swirl flow in the primary and secondary combustion chambers 47, 57.