A data processing device includes (a) a display unit, (b) a light-emitting unit which illuminates the display unit, (c) a communication unit which makes radio communication with other data processing devices, (d) a detector which detects whether the communication unit is in operation or not, and (e) a controller which limits a current to be supplied to the light-emitting unit in accordance with transmission power consumed in radio communication carried out by the communication unit, when the detector has detected that the communication unit is in operation.