An image forming apparatus includes a first and second developing devices for developing an electrostatic images with different developers a toner content detecting device for detecting a toner content in the developer in the first developing device, an image density detecting device for detecting an image density of a reference toner image formed using the first and second developing devices, and a controller for operating the apparatus selectively in a first and second modes. An image forming operation is carried out using only the first developing device in the first mode. An image forming operation is carried out using the first and second developing devices and the in the second mode.