An image forming apparatus includes an image bearing member for bearing an electrostatic latent image; a developing device for developing the latent image on the image bearing member with a developer including toner and carrier particles at a developing position, the developing device including a developer carrying member for carrying the developer, a plurality of developing devices containing toner particles which have colors different from each other, and a moving mechanism for carrying the developing devices to move a selected one of the developing devices to the developing position. The apparatus further includes a density detector for detecting a toner density on the developer carrying member the devices located at a position different from the developing position; a controller, responsive to an output of the density detector, for controlling toner contents in the developing devices. When the density operation in an operation mode, in which only one of the developing devices is used, the controller effects the density detecting operation after the developer carrying member is rotated through a predetermined period of time, for the developing device which comes to a detecting position by way of the developing position.