A boiler 7 includes: a combustion furnace 100 having a combustion chamber 100F therein; a heat exchanger 140 configured to transfer, to water, thermal energy obtained by burning a solid fuel in the combustion chamber 100F; a belt conveyor 120 including a movable loader 120a, disposed on a bottom of the combustion chamber 100F, and configured to move the solid fuel on the movable loader 120a in the combustion chamber 100F; and a fuel supplier 130 connected to a fuel supply port 111a that communicates an inside and an outside of the combustion furnace 100. The belt conveyor 120 has a starting point below the fuel supply port 111alb . The fuel supplier 130 includes a thickness adjuster 131 that adjusts a thickness of the solid fuel on the movable loader 120a of the belt conveyor 120.