There is provided a user apparatus of a radio communication system supporting D2D communication, the user apparatus including a receiver configured to receive a signal of an external synchronization source, a synchronization signal transmitted from a base station, or a synchronization signal transmitted from another user apparatus; and a synchronization processor configured to execute, in accordance with priority information indicating priority order of signals used for synchronization, radio frame synchronization and frequency synchronization using any one of the signal of the external synchronization source, the synchronization signal transmitted from the base station, or the synchronization signal transmitted from the other user apparatus, which is received by the receiver.