a differential apparatus comprises first and second rotary devices rotatable with respect to each other , a viscous clutch having a chamber filled with a viscous fluid and restricting a differential operation generated between the first and second rotary devices by the resistance of the viscous fluid , a friction clutch separated from the viscous clutch and restricting the differential operation , and a device for preventing the seal of the chamber from being reduced by an increase in pressure within the chamber and actuating the friction clutch in accordance with an input torque .