A toner sensor measures the amount of toner attached onto a photosensitive drum upon development by a developing unit. A controller compares the measured toner attaching amount with a predetermined reference value, and executes processing for changing at least one of image forming conditions, for example, the charging amount by a charger, the developing bias voltage of the developing unit, the exposure amount of an optical system, and the toner density of the developing unit, on the basis of the comparison result. Besides the above, a shielding member is provided and is opened only during a measurement mode of the toner sensor in order to prevent the toner sensor from being contaminated by toner during a non-measurement mode.