An apparatus for checking mechanical component parts, such as tools in machine tools, uses optical devices for emitting and receiving a light beam, for example a laser beam, and sensors for detecting the interruption of such light beam. A protection device for at least one of the optical devices includes pneumatic conduits and a nozzle outputting an air stream from a plurality of holes located around the central conduit through which the light beam passes, so generating a tubular shield which embraces the light beam. The protection device further includes a shutter which can be displaced from a rest position, wherein the central conduit is closed and the inside of the protection device is pressurized, to a working position, wherein the nozzle delivers the air stream in the form of a tubular shield. Passing from the rest position to the working position, the shutter assumes an intermediate transit position in which an air blast is outputted through the central conduit for a short time interval. The shutter has cylindrical surfaces and slides inside a shell of the protection device, driven by plane antirotation surfaces, too.