A reflector-mirror drive shaft controller is provided for a laser beam machine which optimizes a beam for workpiece thickness by correcting a control axis of a laser reflector-mirror of a laser beam machine. A laser beam outputted from a laser oscillator device of a laser beam machine is reflected by a reflector-mirror via an output mirror and introduced into a laser machining tool. When a machining head moves along an X axis, the reflector-mirror is moved along a U axis parallel to the X axis to keep optical path length constant. By giving an offset .alpha. to U axis control, the incident beam diameter D to the machining lens is varied to form an optimum machining beam for workpiece thickness.