Provided is a boiler structure with which, by reducing the pressure drop in boiler evaporation tubes correspondingly to the heat flux, which varies in accordance with the distance in the boiler height direction, it is possible to reduce auxiliary power for a water feed pump and so forth, in addition to improving the flow stability and the natural circulation characteristics. The boiler structure includes a number of boiler evaporation tubes that are arranged on a wall surface of a furnace and that form a furnace wall, water pumped into the boiler evaporation tubes being heated in the furnace during flowing inside the tubes to produce steam, wherein the boiler evaporation tubes are formed by connecting tubes of a plurality of types, in which tube wall thicknesses are adjusted on the basis of furnace heat flux such that the higher the furnace heat flux in a region is, the smaller the tube inner diameter becomes.