Abstract: Irradiation treatment of an effluent gas by means of electron beams can be carried out efficiently by controlling the velocity and the velocity distribution of the stream of gas when it passes through a zone to be irradiated. Such control can be effected conveniently according to the present invention by first introducing the stream of gas into a velocity reduction zone and then leading it to a velocity re-accelerating and velocity distribution adjusting zone immediately before the thus adjusted stream of gas reaches the zone to be irradiated.An apparatus suitable for practicing such process is also disclosed.