Abstract: There is provided a portable storage medium, based on USB standard, connected to a USB port of a host computer applying UFI protocol. The portable storage medium comprises a USB connector through which the storage medium is physically connected to the USB port of the host computer; at least one nonvolatile flash memory for storing data transmitted from the host computer; a program storage for storing a predetermined operation program based on USB and UFI; and a controller for controlling the entire operation of the storage medium based on the operation program stored in the program storage.