In a structure comprising host section performs a processing associated with a communication with an outside, and an engine section executes a predetermined function under a management of the host section, a mobile communication terminal is built compactly, performing a convenient key input data processing for a user. When key input data is notified to a host section 30 from an operating section 21 by the user, the host section judges whether or not the key input data is input by operating a host-specific key is made. If the result is affirmative, the host section 30 judges whether or not the engine section 40 is in an engine mode to be notified of the key input data, estimating the engine mode is continued. Further, if the result is affirmative, the host section 30 notifies the key input data of the engine section 40.