The mobile communication terminal has a main body unit and an attachment. When the main body unit separates from the attachment, the attachment transmits a predetermined radio signal. The main body unit receives the radio signal and determines whether its distance from the attachment exceeds specified distance. When the main body unit is at a distance from the attachment exceeding the specified distance, predetermined operation of the main body unit, for example, entering operation using a keypad, memory read-out operation, data transmission and data reception, are disabled. In this manner, it is possible to prevent unauthorized use by third parties or information leaks to third parties when the main body unit has been lost.