Abstract: An IC card is provided with a nonvolatile data area in which data can be read and written, a nonvolatile key data area into which data can be written but cannot be read, and a nonvolatile key register into which data can be written but cannot be read. Writing predetermined data, which are not encrypted but need to be kept secret, into the data area and key data into the key data area, inhibits reading and writing data in the data area. Reading or writing in the data area is authorized only if proper key data are written into the key register, and inhibited if improper key data are written into the key register.