An image forming apparatus is provided with an original density detecting device (4) for detecting the original density, and output characteristic switching means (1, 6) for switching the output characteristic relative to the original density detected by the original density detecting device (4) to a plurality of types of predetermined output characteristics (a, b, c). The original density is found on the basis of a predetermined output characteristic out of the plurality of output characteristics of the original density detecting device (4) and an output of the original density detecting device. Then, an output characteristic showing a high rate of change in the vicinity of the original density found by the first original density measuring means out of the plurality of output characteristics is selected by selecting means (1). When the selected output characteristic is different from the output characteristic used for measuring the original density by the first original density measuring means, the output characteristic of the original density detecting device (4) is switched to the selected output characteristic, and the original density is found on the basis of the selected output characteristic and the output of the original density detecting device (4) by second original density measuring means.