A mobile terminal device, which is provided with the plurality of keys, includes: a touch detection interface for detecting that a user has touched a key; a press detection interface for detecting that the user has pressed the key; a controller for switching the mode in response to whether or not the user has touched a side key, in order to execute a first function assigned to the key in a case where the key is pressed by the user in a “shift mode,” and to execute a second function assigned to the key in a case where the key is pressed by the user in a “normal mode”; and a display unit for displaying an explanation on the first function assigned to the key in a case where it is detected in the “shift mode” that the user has touched the key.