An apparatus for developing a latent image formed on a photoreceptor constituted a container for storing developer, a toner feeder for replenishing toner, a sensor for measuring the toner concentration in the developer, a memory for storing a reference measurement lying only with a certain range and which is obtained by measuring a reference developer having a reference toner concentration by the sensor, and a controller for controlling a toner concentration of the current developer by operating the toner feeder so that a current measurement obtained by the sensor becomes equal to the reference measurement.