To provide a portable electronic device preventing illegal exploitation of stored data under a state that a use of a transaction function is inhibited (invalidated), and a method for inhibiting the use.

At the time of receiving a card block command from the outside (ST21), a control element 15 verifies MAC of the received card block command (ST22). As a result of the verification, in the case that the MAC is a proper value, an LSB of state management information of a data memory 16 is updated from '1(usable state)' to '0(unusable state)' (ST24). When updating is ended, all input/output data 130 of the data memory 16 are erased (ST25). When updating of the state management information and erasure of the data are normally ended, the control element 15 outputs a status '9000' indicating normal end to the outside, ends a series of function invalidating processings and an IC card 1 becomes unusable thereafter (ST26).