An illumination sensing device for a portable terminal is disclosed. The illumination sensing device includes a housing having a sensor hole extending through a side of the housing, a printed circuit board in the housing and connected to a backlight unit, an illumination sensor coupled with the printed circuit board and disposed in the sensor hole, and an optical guide disposed in the sensor hole and facing the illumination sensor. The illumination sensor senses light introduced through the optical guide from outside the housing, and the backlight unit is operated based on the light sensed by the illumination sensor.