A developer detector includes a sensor housing portion, an optical sensor, and a cleaning portion. The sensor housing portion can be installed on a wall surface of a hopper storing a developer. The sensor housing portion includes a detection surface, and a step surface protruding a smaller amount from the wall surface than the detection surface to form a step height from the detection surface. The optical sensor is housed in the sensor housing portion and detects the developer through the detection surface. The cleaning portion moves in a moving region including a first region facing the detection surface and a second region facing the step surface and cleans the detection surface on the way of moving. The cleaning portion includes a flexible member and makes the flexible member slide on the detection surface while making the flexible member contact the detection surface under pressure in the first region.