An apparatus includes a system controller which calculates a ratio of a first signal to a second signal and adjusts a control parameter of an electrophotographic reproduction process so that the density of fog indicated by the ratio is within a predetermined range. A first optical sensor outputs the first signal by detecting a reflectance of a non-image area on the photoreceptor between a developing unit and a transfer charger. A second optical sensor outputs the second signal by detecting a reflectance of a surface of the photoreceptor between a cleaning unit and a charger. The system controller controls developer including toner and carrier to be agitated when the ratio obtained after adjusting the control parameter is still outside the predetermined range. Since the density of fog is detected with accuracy by calculating the ratio, copy quality is stabilized even when temperature changes. Moreover, even if the ratio is outside the predetermined range as a result of the supply of toner which has deteriorated due to a heat history, the deterioration of toner is restored by agitating the developer. It is thus possible to prevent copy quality from being degraded.