Data stored in a memory unit can be transmitted to an external apparatus such as a PC, a television or a printer without relying on operations carried out on a host apparatus, onto which the memory unit has been mounted. The memory unit includes a memory card and an adapter into which the memory card is inserted. At a request made by the external apparatus through a radio transmission line as a request for data, the data can be read out from a memory device employed in the memory card in accordance with control executed by a communication control section employed in the adapter. Thus, it is not necessary to install driver software for controlling communications in a digital camera or the like serving as the host apparatus, on which the memory unit is mounted.