a developing apparatus which includes a toner replenishing device for replenishing toner into the developing apparatus , a toner concentration detecting sensor for generating an output coresponding to the toner amount in a developing material stirred and transported through the developing apparatus , with the output containing a varying output periodically produced and stable output , a discriminating element for deriving the stable output of the outputs from the toner concentration detecting element , and a comparator for outputting a feeding signal to the toner replenishing device through comparison between the output from the discriminating element and a preliminarily set reference value .