A controller controls a first image forming unit and a second image forming unit to form a test image having a pattern. The test image is formed by the first image forming unit based on a first image forming condition in which an absolute value of a developing potential of a first developing sleeve is greater than an absolute value of a charging potential of a first photosensitive member. The pattern is formed by a second image forming unit based on a second image forming condition in which an absolute value of a developing potential of a second developing sleeve is smaller than an absolute value of a charging potential of a second photosensitive member. A controller controls a sensor to read the test image having the pattern.