An image forming apparatus includes a line sensor capable of reading a sheet being conveyed. A signal processing circuit generates, based on respective analog picture signals of pixels and in order of sequence of the pixels, detection signals each indicating whether or not a reflecting object has been read. A first counting circuit measures a duration time which is a time during which the detection signals maintain a level indicating that the reflecting object has been read. When the duration time reaches or exceeds a first specified time, the first counting circuit changes a level of a first detection result signal to a level indicating necessity of cleaning. A unit control circuit determines, based on the level of the first detection result signal, whether or not cleaning is necessary.