An image forming apparatus includes an image bearing member (25) for bearing an electrostatic latent image; developing means (17) for developing the latent image on the image bearing member with a developer including toner and carrier particles at a developing position, the developing means including a developer carrying member (3) for carrying the developer, a plurality of developing devices (1K,1Y,1M,1C) containing toner particles which have colors different from each other, and moving means (18) for carrying the developing devices to move a selected one of the developing devices to the developing position,
   the apparatus further including density detecting means (91) for detecting a toner density on the developer carrying member (3) of the devices located at a position different from the developing position; control means (50), responsive to an output of the density detecting means (91), for controlling toner contents in the devices (1Y,1M,1C), wherein when the density detecting means effects its density detecting operation in an operation mode, in which only one of the devices is used, the control means effects the density detecting operation after the developer carrying member is rotated through a predetermined period of time, for the device which comes to the detecting a position by way of the developing position.