In an apparatus for mixing a fluid with a large gas stream (2) flowing in a gas duct, especially for introducing a reducing agent (5a) into a flue gas comprising nitrogen oxides, having at least one metering lance (5) with at least one atomizer nozzle (4a; 4b) for the feeding of the fluid, whose axis forms an angle with the flow direction of the gas stream, and at least one flat mixer element which is arranged at a distance from the nozzle and forms an angle with the flow direction of the gas stream, wherein flow eddies (3) are formed at the mixer element and at least some of the fluid gets into these flow eddies, deposit formation is prevented to a substantial extent in the case of direct injection of a liquid as the fluid, especially of a liquid reducing agent, with a short mixing path, in accordance with the invention by providing that, in the case of use of a liquid (5a) as the fluid,; the metering lance (4) is equipped with at least two atomizer nozzles (4a, 4b) which are tilted against the flow direction of the gas stream (2) and in opposite directions to one another, such that the atomizer nozzles are assigned to a mixer element (1) with a disk-like design.