A developer amount detector for detecting an amount of developer contained in a unit removably installable in an image forming apparatus, including optical elements having a light-emitting element to emit light and a light-receiving element to receive the light; two light-guiding members, provided in the unit, to guide the light from the light-emitting element to the light-receiving element; a holder, held in the image forming apparatus, to hold the optical elements, movable in a direction orthogonal to optical axes of the optical elements as the unit is being installed in the image forming apparatus; and a positioning mechanism to position the unit within the image forming apparatus by restricting movement of the holder in the direction orthogonal to the optical axes relative to the unit in a state in which the unit is installed in the image forming apparatus.