A mobile communication terminal device whose authentication and settlement functions by noncontact proximity communication can be continuously used even after operating voltage from battery power drops is provided. Only when the supply of required power from a battery is lost, a security controller is controlled into a mode in which it operates with low power consumption and noncontact authentication and settlement functions are ensured by external electromagnetic field power. Thus the noncontact authentication and settlement functions can be used even after the battery remaining capacity is lost by use of a communication function for the principal purpose. Specifically, the following is implemented: when there is the supply of required power from the battery, it is made possible to carry out high-performance, multifunctional authentication and settlement processing making good use of high-speed processing, mass storage, and the like which are the advantages of the security controller essentially driven by battery; and in an anomalous instance in which the battery remaining capacity is lost, it is made possible to carry out minimal authentication and settlement processing.