PURPOSE: To write and read-out at a high speed, by providing a connecting terminal for connecting an external storage device provided on an information processing device, and an RAM having an electric power supplying circuit to be connected to this terminal, a buffer and a write controlling circuit, with information holding function.

CONSTITUTION: A connecting terminal 2 of an external storage device 1 provided on an information processing device is connected to the external storage device body 1 through an electric power supply line 4, a chip select line 5, an address bus 6 and a data bus 7. On the body 1, an electric power supplying circuit 8, an address buffer 11, a data buffer 12, a write controlling circuit 13 and a changeover switching circuit 15 are provided, and information from the information processing device is written and read out to and from an RAM9 connected through the buffer. Even in case when the connecting terminal 2 is detached from the information processing device, storage of the RAM is not erased since it is backed up by the electric power supplying circuit 8. In this way, an exclusive interface is not requied, and said processing is executed at a high speed.