Image forming apparatus and controlling method therefor

An image forming apparatus includes a first station including a first photosensitive drum, a second station including a second photosensitive drum, and an intermediate transfer member for receiving the first toner image from the first drum and the second toner image from the second drum sequentially. In addition, a first sensor detects a first index image on the intermediate transfer member, and a second sensor detects a second index image on the second drum. A controller controls a peripheral speed of the second drum, and an executing device executes a test mode using a first test inclined index image and a second test inclined index image. The controller controls image forming conditions for the first and second index images in accordance with outputs of the first and second sensors in the test mode.