When a control data stored in a memory is changed, an operation mode of a mobile communication terminal is set according to an input of a security code to a registration preparing state where a lock code can be registered. Next, in the registration preparing state, the input lock code is registered in the memory, and setting the operation mode to a protective state where the stored control data cannot be changed from outside. Then, when a lock release code corresponding to the lock code is input with the protective state being set, the protective state is canceled temporarily, thereby enabling the stored data to be changed. After the change of the stored control data, the registration of the lock code is deleted, and restoring the registration preparing state.