Abstract: The disclosure is directed to an improved optical unit provided with a driving shaft which is arranged to be displaced in an axial direction for connection with a driven shaft of a mating instrument, following attachment or detachment with respect to the mating instrument. In the optical unit, safety of the mounting surface of the mating instrument and safety of the connecting end portion of the driving shaft have been ensured by providing a difference between the amount of displacement of a position restricting member advancing into a mounting position restricting recess provided in a mounting surface of the mating instrument, and the amount of displacement of the driving shaft to be connected to an end to be clutched of a driven shaft which is borne or journalled in a hole formed in the mounting portion of the mating instrument.