To provide a boiler for reliably and efficiently burning biomass fuel such as bagasse, waste, bark, palm cake or chaff or solid fuel consisting of coal and biomass fuel mixed and burnt, without being greatly influenced by a water content or grain sizes.

The boiler for burning the biomass fuel or the solid fuel consisting of coal and biomass fuel mixed and burnt comprises a combustion chamber covered with a furnace wall for storing the solid fuel, and a first fire grate and a second fire grate for burning the solid fuel stored in the combustion chamber. The first fire grate is provided on the bottom of the combustion chamber, the lower part of the furnace wall is an inclined portion inclined so that the cross section area is gradually smaller toward the bottom, and the second fire grate is arranged along the inclined portion.