An image forming apparatus has a developer container containing a developer and a developer carrying member for carrying the developer and adapted to switch an AC voltage to be applied to the developer carrying member. The apparatus includes a detecting member for detecting the amount of developer in the developer container and a processing portion for determining the amount of developer in the developer container according to the value output by the detecting member when the AC voltage is applied to the developer carrying member. The processing portion is adapted to determine the amount of developer by executing a process that corresponds to the operation of switching the AC voltage.