An image forming apparatus includes a sensor having a light emitting section for irradiating a detection area and a light receiving section for outputting a light sensitive signal indicative of the amount of the light received from the detection area. A sensor adjustment section adjusts the sensitivity of the sensor by varying the set value of an adjustable characteristic value associated with the sensitivity, based on explorative measurement of the adjustable characteristic value corresponding to a predetermined reference level of the light sensitive signal. If an estimation section estimates that the amount of a leakage current generated on the light receiving section is larger than a reference amount, a mitigation section mitigates the influence of the leakage current on the explorative measurement, by at least one of adjustment of the light receiving section for reduction of the leakage current and modification of the predetermined reference level.