In a waste gas treating method wherein waste gas is treated by adding ammonia thereto and irradiating this waste gas with electron beams, the adhesion of a by-product to the inner wall of a duct (9) is inhibited by feeding the waste gas after irradiation with electron beams at a flow velocity of 10m/sec or less, more preferably 5 m/sec or less, until it at least reaches a first by-product collector (11). The duct may have a rectangular cross-sectional configuration. Thus, it is possible to inhibit increase in the pressure loss of the waste gas being treated and enable stable operation of the waste gas treating process.
