An image forming apparatus uses a plurality of image forming units that are arranged rotatable in a vertical plane such that the image forming units are shifted sequentially to an image forming position. Each image forming unit has a translucent toner detection window on its outer periphery for detecting the remaining amount of toner in a toner hopper. The image forming units will have a replacing position in which it will face an opening for allowing replacement of the image forming unit. The translucent detection window will face the opening when the image forming unit is in the replacing position.