A solid-fuel burner provided in a wall of a furnace comprising: a fuel nozzle that is opened in a boiler furnace wall surface having a solid-fuel passage connected to a cylindrical fuel carrier piping through which a mixed fluid of a solid fuel and a carrier gas for the solid fuel flows; and combustion gas nozzles (10, 15) that communicate with a wind box in which a combustion gas for the solid fuel flows and are formed on an outer peripheral wall side of the fuel nozzle, wherein the solid-fuel burner comprises in the fuel nozzle: a venturi having a constricting portion that reduces a transverse cross section of the solid-fuel passage in the fuel nozzle; and a fuel concentrator that diverts a flow in the nozzle outward on a wake side of the venturi, and the fuel nozzle is formed so that (a) an opening shape thereof near an opening portion in the boiler furnace wall surface is a flat shape, (b) a cross-sectional shape thereof orthogonal to a nozzle center axis on the outer peripheral wall of the fuel nozzle is a circular shape in a transverse cross section from a burner introducing portion of the fuel carrier piping to the constricting portion of the venturi, characterized in that (c) a portion where a degree of flatness defined by a ratio of W/H, where W is the long diameter or long side and H is the short diameter or the short side, of a cross section orthogonal to a nozzle center axis of the fuel nozzles gradually increases is provided between the constricting portion of the venturi and the opening portion provided in the boiler furnace wall surface, and (d) the opening portion in the boiler furnace wall surface is formed into a flat shape having the maximum degree of flatness. a fuel nozzle that is opened in a boiler furnace wall surface having a solid-fuel passage connected to a cylindrical fuel carrier piping through which a mixed fluid of a solid fuel and a carrier gas for the solid fuel flows; and combustion gas nozzles (10, 15) that communicate with a wind box in which a combustion gas for the solid fuel flows and are formed on an outer peripheral wall side of the fuel nozzle, wherein the solid-fuel burner comprises in the fuel nozzle: a venturi having a constricting portion that reduces a transverse cross section of the solid-fuel passage in the fuel nozzle; and a fuel concentrator that diverts a flow in the nozzle outward on a wake side of the venturi, and the fuel nozzle is formed so that (a) an opening shape thereof near an opening portion in the boiler furnace wall surface is a flat shape, (b) a cross-sectional shape thereof orthogonal to a nozzle center axis on the outer peripheral wall of the fuel nozzle is a circular shape in a transverse cross section from a burner introducing portion of the fuel carrier piping to the constricting portion of the venturi, characterized in that (c) a portion where a degree of flatness defined by a ratio of W/H, where W is the long diameter or long side and H is the short diameter or the short side, of a cross section orthogonal to a nozzle center axis of the fuel nozzles gradually increases is provided between the constricting portion of the venturi and the opening portion provided in the boiler furnace wall surface, and (d) the opening portion in the boiler furnace wall surface is formed into a flat shape having the maximum degree of flatness. (a) an opening shape thereof near an opening portion in the boiler furnace wall surface is a flat shape, (b) a cross-sectional shape thereof orthogonal to a nozzle center axis on the outer peripheral wall of the fuel nozzle is a circular shape in a transverse cross section from a burner introducing portion of the fuel carrier piping to the constricting portion of the venturi, characterized in that (c) a portion where a degree of flatness defined by a ratio of W/H, where W is the long diameter or long side and H is the short diameter or the short side, of a cross section orthogonal to a nozzle center axis of the fuel nozzles gradually increases is provided between the constricting portion of the venturi and the opening portion provided in the boiler furnace wall surface, and (d) the opening portion in the boiler furnace wall surface is formed into a flat shape having the maximum degree of flatness.