The invention provides a focus adjuster for laser beam machine which adjusts focus by detecting reflected light of a laser beam in a laser beam machine.A laser beam outputted from a laser oscillator device 550 is reflected by a mirror 570 via an output mirror 560 and introduced into a laser machining tool. The laser beam collected by a machining lens 62 is reflected by a reflector plate M1 and sensed by a reflection detector 580. Automatic focus adjustment is achieved by sensing output of the reflected light as well as changes in focal position due to contamination of the machining lens 62.