A toner-amount detection sensor includes a light-emitting element, a first light-receiving element, and a toner-amount calculating unit. The light-emitting element emits light toward the surface of the transfer body at a predetermined incident angle. The first light-receiving element is disposed on a side opposite to the light-emitting element with respect to a plane normal to the surface of the transfer body. The first light-receiving element receives light reflected from the surface of the transfer body. The toner-amount calculating unit calculates the amount of toner from the quantity of the reflected light received by the first light-receiving element. The sensor has a relationship of A1