An IC memory card includes an input/output unit attachable to or detachable from a host processing apparatus, a storage unit having a semiconductor memory for storing data, and a control unit for performing write control of data transmitted from the host processing apparatus into the storage unit and read control of the data stored in the storage unit, which is to be transferred to the host processing apparatus. In such an IC memory card, the storage unit is provided with a fixed area in which fixed information not to be overwritten has previously been written and with a variable area in which rewritable data and management information for the rewritable data are written. The control unit includes an address detector for detecting if the fixed area of the storage unit is accessed and for generating a detection signal if such an access occurs. A read/write controller is included for providing such a control that when the address detector detects the fact that the fixed area of the storage unit is accessed, a control signal for writing of data supplied from the host processing apparatus is masked by the detection signal so as to inhibit the data from being written into the storage unit.