In a laser machining system for the heat treatment of an object, a pair of cylindrical concave mirrors are used to receive a source laser beam and irradiate a linear laser beam on to the object. The system is further provided between the latter cylindrical mirror and the object with at least a pair of slant-type plane mirrors for conditioning the linear beam. The plane mirrors have means for variably making the lengthwise dimension of the output linear beam smaller than that of the incident linear beam, so that a linear laser beam of various lengthwise dimensions can be produced.