A blocking disk formed with a notched part is disposed between a density sensor and a belt. The exposure/blocking of the density sensor relative to the belt is switched by rotating the blocking disk. In one example, a disk-supporting axel gear is formed on a disk-supporting axel that is the axis of rotation of the blocking disk, where the disk-driving gear is supported on a sensor frame so as to be able to rotate and so as to mesh with this disk-supporting axel gear. In one example, the disk-driving gear is connected to a driving force transmission mechanism for rotationally driving a belt-driving roller, where, when the belt-driving roller is driven rotationally, the blocking disk is constantly rotated by a disk-driving gear.