PURPOSE: To prevent pressure loss, in a gas mixing apparatus used in denitration, by mounting the gas mixing chamber provided to the lateral side of an exhaust gas line, a gas mixing plate arranged in a gas stream in a screen like state and a forcibly moving and stopping apparatus for applying slide movement to said gas mixing plate.

CONSTITUTION: At the time of the low load operation of a boiler, a gas mixing plate 8 is fallen to block an exhaust gas line region 11 to guide exhaust gas, after low temp. gas from an exhaust gas line 6 and high temp. gas from an exhaust gas extraction line 5 were met, to a gas mixing chamber 7 and the mixing of both gases is performed. At the time of high load operation, the gas mixing plate is upwardly drawn into the gas mixing chamber 7 by an electromotor through a screw rod 14 and the exhaust gas line region is opened to allow exhaust gas to straightly pass the exhaust gas line and the obstruction of the exhaust gas stream is removed to contrive reduction of pressure loss.