A mobile terminal device, which is provided with the plurality of keys, includes: a detection interface for detecting touching any of the keys by a user; a press detection interface for detecting pressing the key by the user; a controller for executing a first function assigned to the key at the time of detection of the user touching the key in a case where touching time period, for which the user touches the key, is equal to, or longer than, a predetermined judging time, and for executing a second function assigned to the key at the time of detection of the user touching the key in a case where the touching time period is shorter than the predetermined judging time; and an output controller for displaying an explanation on the first function on a display unit in a case where the touching time period is equal to, or longer than, the predetermined judging time.