An image forming apparatus includes a rotatable member which is provided on and rotatable about a rotation shaft in a detachably mountable developing unit containing a developer and has flexibility so that it is flexed depending on a resistance of the developer; an electroconductive member to be detected which is provided on the rotatable member; a detecting electrode provided in a neighborhood of an outer wall surface of a bottom of the developing unit; a converting portion for detecting an electrostatic capacity between the member to be detected and the detecting electrode and for converting the electrostatic capacity into an electric signal; a measuring portion for measuring a time duration in which the electric signal converted by the converting portion exceeds a predetermined threshold; and a discriminating portion for discriminating an amount of the developer on the basis of the time duration measured by the measuring portion.