Abstract: In a power transmission apparatus of viscous coupling type, when two first and second output shafts are kept rotated at two big different speeds for many hours as when the ordinary tire is replaced with a spare tire, temperature and pressure within viscous fluid rise abnormally high. In this case, viscous coupling limit slip differential mode is switched to a free differential mode by means of a movable clutch shaft. Further, it is also preferable to directly couple the first and second output shafts to obtain a differential lock mode when the vehicle is travelling on muddy road.