An image forming apparatus, comprising: a plurality of photosensitive bodies (28K, 28Y, 28M, 28C) disposed along a moving direction of a recording medium; a drive mechanism configured to drive and rotate the plurality of photosensitive bodies (28K, 28Y, 28M, 28C); and a plurality of exposure units, each exposure unit being associated with a respective one of the photosensitive bodies, the each exposure unit configured to expose the respective photosensitive body; a registration sensor configured to detect a leading edge of the recording medium; a determining unit configured to determine a time after the detection timing at which the registration sensor detects the leading edge of the recording medium as an exposure-starting timing of one photosensitive body and to determine an exposure-starting phase that is a rotation phase at the exposure-starting timing with respect to the one photosensitive body; a varying unit configured to store information regarding a corresponding relationship between the rotation phase of the one photosensitive body and the exposure-starting timing of the one photosensitive body and an exposure-starting timing of an other photosensitive body disposed at a downstream side of the one photosensitive body in the moving direction of the recording medium, and to vary an exposure-starting time difference (ΔT') between the exposure-starting timing of the one photosensitive body and the exposure-starting timing of the other photosensitive body, based on the information regarding the corresponding relationship according to the exposure-starting phase determined by the determining unit and wherein the drive mechanism includes a common drive source for driving and rotating the one photosensitive body and the other photosensitive body characterized by the determining unit including a reference rotation phase sensor configured to detect that the photosensitive body is brought into a reference rotation phase, and the determining unit determines the exposure-starting phase based on a detection timing of the reference rotation phase sensor and the exposure-starting timing of the one photosensitive body. a plurality of photosensitive bodies (28K, 28Y, 28M, 28C) disposed along a moving direction of a recording medium; a drive mechanism configured to drive and rotate the plurality of photosensitive bodies (28K, 28Y, 28M, 28C); and a plurality of exposure units, each exposure unit being associated with a respective one of the photosensitive bodies, the each exposure unit configured to expose the respective photosensitive body; a registration sensor configured to detect a leading edge of the recording medium; a determining unit configured to determine a time after the detection timing at which the registration sensor detects the leading edge of the recording medium as an exposure-starting timing of one photosensitive body and to determine an exposure-starting phase that is a rotation phase at the exposure-starting timing with respect to the one photosensitive body; a varying unit configured to store information regarding a corresponding relationship between the rotation phase of the one photosensitive body and the exposure-starting timing of the one photosensitive body and an exposure-starting timing of an other photosensitive body disposed at a downstream side of the one photosensitive body in the moving direction of the recording medium, and to vary an exposure-starting time difference (ΔT') between the exposure-starting timing of the one photosensitive body and the exposure-starting timing of the other photosensitive body, based on the information regarding the corresponding relationship according to the exposure-starting phase determined by the determining unit and wherein the drive mechanism includes a common drive source for driving and rotating the one photosensitive body and the other photosensitive body characterized by the determining unit including a reference rotation phase sensor configured to detect that the photosensitive body is brought into a reference rotation phase, and the determining unit determines the exposure-starting phase based on a detection timing of the reference rotation phase sensor and the exposure-starting timing of the one photosensitive body.