An image forming apparatus includes a color difference detection pattern output device that outputs an image signal to produce a color difference detection pattern for detecting periodic rotational variations arising in the image forming apparatus, a pattern detection device for detecting the color difference detection pattern formed on an endless carrier; phase detection device for detecting the phase of rotation of at least one of the image carriers of the image forming apparatus on the basis of a detection signal received from the pattern detection device; and a rotational phase control device for individually controlling the phase of rotation of at least one of the image carriers of the image forming apparatus and the endless carrier on the basis of the phase information detected by the phase detection device. The phase detection device calculates the mean value of the rotational variations of the rotational phases.