To provide an electrophotographic image forming apparatus capable of detecting conditions apt to cause scumming, in advance, and decreasing toner concentration, accordingly, thereby preventing occurrence of the scumming.

In the image forming apparatus, a reference pattern is formed on an image carrier, the reflected density of the reference pattern is detected, and then, the toner concentration in a developing device is controlled based on the detection result, and also, the apparatus includes a function to count an image forming operation period for every image forming job and a function to count the number of printed sheets. Provided that the image forming operation period is expressed by A (second) and the number of printed sheets is expressed by B (page), the toner concentration is controlled in accordance with A/B as a calculated value.