An apparatus for detecting average picture level (APL) which is able to prevent non-linear property of APL used to control image and power in a PDP system, to calculate linear APL value, and to calculate gain value automatically, comprises: a pre-processing unit for summing image data of one frame, multiplying the summed value by weight corresponding to a size of pixel, and outputting the data multiplied by the weight; an average calculating unit for summing upper bits of the data outputted from the pre-processing unit and outputting the summed value as an average value; a gain calculating unit for calculating gain for obtaining maximum APL value based on the average value outputted from the average calculating unit; and an APL outputting unit for multiplying the gain obtained from the gain calculating unit by the average value outputted from the average calculating unit, and outputting the multiplied value as a final APL value.