A denitration apparatus for a coal fired boiler, the denitration apparatus comprising: a curved duct having a duct inlet, a duct outlet, and a bend section whose portion of a flow channel through which exhaust gas that flows in from the duct inlet and flows toward the duct outlet passes, is curved, a flow channel direction of the duct outlet being a horizontal direction; and a catalytic reactor connected to the duct outlet of the curved duct, whereby the curved duct and the catalytic reactor are integrated, wherein the catalytic reactor has a flow channel expanded part whose flow channel of the catalytic reactor is expanded such that a flow direction of exhaust gas that flows in from the duct outlet and passes through the flow channel expanded part is changed from the horizontal direction to a vertical direction, wherein the catalytic reactor immediately downstream of the flow channel expanded part has a section whose flow channel cross-sectional area is substantially constant, whose flow channel cross-sectional area is approximately three times as large as a flow channel cross-sectional area of the curved duct, and which is provided with a rectifying device with multiple parallel flat plates, ammonia injection nozzles for jetting ammonia gas, and a catalyst layer, wherein the rectifying device and the ammonia injection nozzles are disposed at an inlet portion of the section of the catalytic reactor whose flow channel cross-sectional area is substantially constant and are located upstream of the catalyst layer, wherein the ammonia injection nozzles are set immediately downstream of the rectifying device in such a way as to be continuous with the rectifying device, wherein the ammonia injection nozzles are each provided with a tube, and with nozzles which extend downward from the tube, the tube being circular in a cross section perpendicular to a longitudinal direction of the tube, the tubes being further installed in such a way as to be spaced apart from each other at predetermined intervals in the horizontal direction and as to have their longitudinal direction perpendicular to the horizontal direction and to the vertical direction, wherein ammonia gas introduced into the tubes is jetted into the exhaust gas via the nozzles extending downward, wherein the parallel flat plates are installed in such a way as to be spaced apart from each other at predetermined intervals in the horizontal direction, such that the flows of the exhaust gas passing through the parallel flat plates become uniform due to a flow channel resistance of the parallel flat plates, and wherein each of the flat plates of the rectifying device is oriented such that a side oriented in the vertical direction is a short side and a longitudinal direction is perpendicular to the horizontal direction and to the vertical direction. a curved duct having a duct inlet, a duct outlet, and a bend section whose portion of a flow channel through which exhaust gas that flows in from the duct inlet and flows toward the duct outlet passes, is curved, a flow channel direction of the duct outlet being a horizontal direction; and a catalytic reactor connected to the duct outlet of the curved duct, whereby the curved duct and the catalytic reactor are integrated, wherein the catalytic reactor has a flow channel expanded part whose flow channel of the catalytic reactor is expanded such that a flow direction of exhaust gas that flows in from the duct outlet and passes through the flow channel expanded part is changed from the horizontal direction to a vertical direction, wherein the catalytic reactor immediately downstream of the flow channel expanded part has a section whose flow channel cross-sectional area is substantially constant, whose flow channel cross-sectional area is approximately three times as large as a flow channel cross-sectional area of the curved duct, and which is provided with a rectifying device with multiple parallel flat plates, ammonia injection nozzles for jetting ammonia gas, and a catalyst layer, wherein the rectifying device and the ammonia injection nozzles are disposed at an inlet portion of the section of the catalytic reactor whose flow channel cross-sectional area is substantially constant and are located upstream of the catalyst layer, wherein the ammonia injection nozzles are set immediately downstream of the rectifying device in such a way as to be continuous with the rectifying device, wherein the ammonia injection nozzles are each provided with a tube, and with nozzles which extend downward from the tube, the tube being circular in a cross section perpendicular to a longitudinal direction of the tube, the tubes being further installed in such a way as to be spaced apart from each other at predetermined intervals in the horizontal direction and as to have their longitudinal direction perpendicular to the horizontal direction and to the vertical direction, wherein ammonia gas introduced into the tubes is jetted into the exhaust gas via the nozzles extending downward, wherein the parallel flat plates are installed in such a way as to be spaced apart from each other at predetermined intervals in the horizontal direction, such that the flows of the exhaust gas passing through the parallel flat plates become uniform due to a flow channel resistance of the parallel flat plates, and wherein each of the flat plates of the rectifying device is oriented such that a side oriented in the vertical direction is a short side and a longitudinal direction is perpendicular to the horizontal direction and to the vertical direction.