Abstract: A device for beam machining comprises an extended radiation source (1) and a concentrating reflector (2) whose reflecting surface has the form of a surface of revolution made up of several successively-joined segments (3). The shape of the reflector generating surface on any segment except the initial one located near the apex M of the reflector (2) and the boundaries between the joined segments are defined by a system of equiations. The initial segment of the profile of the reflector (2) extending from its apex M to the profile points R.sub.1, z.sub.1 has the shape of a segment of the curve of the 2nd order. Installed on the end of the concentrating reflector is a pressuretight protective transparent partition (4) whose cross-sectional profile is capable of ensuring additional focusing of preset radiation fractions of the linear source (1) in the focal point F.