A data processing device includes a digital data processing unit and a control unit. The digital data processing unit includes a computing unit that computes digital data and a power source management unit that transceives commands with the computing unit and manages a power supply to the computing unit. The control unit controls a user interface unit that provides a user interface function. The control unit diagnoses an operation of the digital data processing unit by monitoring a transfer state of the commands between the computing unit and the power source management unit. When determining an abnormality occurrence in the operation of the digital data processing unit, the control unit resets all parts of the digital data processing unit or a part of the digital data processing unit without interrupting an operation of the user interface unit.