A pair of indentations is formed with a vertical spacing on a side wall of a toner cartridge, respective translucent windows are arranged on opposing upper and lower surfaces of the indentations protruding into the toner cartridge, and a light-emitting element and a light-receiving element of a remaining toner sensor are inserted to the indentations such that the light-emitting element and the light-receiving element of the remaining toner sensor face each other via the translucent windows of the indentations. When there is sufficient toner remaining in the toner cartridge, there is toner in the space between the transparent plates, and the space between a light-emitting diode and a phototransistor is blocked such that light is not incident on the phototransistor, whereas when there is little toner remaining in the toner cartridge, there is no toner in the space between the transparent plates, and the light is incident on the phototransistor via the space between the light-emitting diode and the photoresistor.