To obtain a portable terminal device which controls a back light of a display part in accordance with the use stat of the device.

When the distance to an obstacle in the front of a display part 5, which is the detection result of a distance sensor 7 which detects this distance, is shorter than a first prescribed value, a control part omitted in figure extinguishes the back light of the display part 5 because it can be judged that a portable telephone set 1 is not in such state that a user can see the display part 5. When the distance is longer than the first prescribed value and is shorter than a second prescribed value, the control part lights the back light because it can be determined that the user is seeing the display part 5 of the portable telephone set 1. When the distance is longer than the second prescribed value, the control part extinguishes the back light because it can be judged that the portable telephone set 1 is left.