A mobile terminal device is provided which can be used without an incongruous feeling. In Step S14, it is determined whether or not the operated application is the second display state (landscape display). When it is determined that the application is not the landscape display, the process is transferred to Step S15, and the key operation functions assigned to the keys b1 to b6 of the “TURN-CLOSE STATE (LENGTHWISE)” of the key assignment table are assigned to the keys b1 to b6. In Step S14, when it is determined that the operated application is the landscape display, the process is transferred to Step S16, the key operation functions assigned to the keys b1 to b6 of “TURN-CLOSE STATE (WIDTHWISE)” of the key assignment table are assigned to the keys b1 to b6, and the key operation unit in the turn-close state has the same function as the cross key 15a.