To provide an inexpensive lower gasification combustion boiler which is simply operated, easily maintained, quickly started up to a normal operation without generating smoke of solid biomass fuel of raw logs, raw fuel wood and raw chips of various sizes and various water content, and without scattering ash, and controlled in temperature while keeping stable high-temperature combustion.

A premixed combustion chamber 4 is disposed between a primary combustion chamber hearth opening portion 2b and a secondary combustion chamber 3 connected to a lower part of the opening portion 2 in this lower gasification combustion structure, an unburned gas from the primary combustion chamber 2 and the secondary combustion air are mixed and burned at a high temperature to be changed to combustion flame free from smoke, the combustion flame and ash Z are discharged to the vertically-long secondary combustion chamber 3 provided with water-cooled walls 8 on its side face and bottom face, and the radiation heat is supplied to the water-cooled walls 8, thus the ash Z is stored on the bottom face, and a temperature of the boiler is controlled by controlling the combustion to be strong and weak.