In an image forming apparatus, a computing unit computes phase shift amounts A(i) for every correction relative phase angle θ(j) based on an amplitude B of a reference compressional wave αa, an amplitude C(i) of a detection compressional wave α(i), and a relative phase angle φ(i). A setting unit specifies the phase shift amounts A(i), and sets a correction relative phase angle θ(j) corresponding to the specified phase shift amounts A(i). A correction unit corrects, based on the correction relative phase angle θ(j) set by the setting unit, a relative phase shift between a periodic variation in the circumferential speed of a first image bearing member and a periodic variation in the circumferential speed of a second group image bearing member by operationally controlling at least one of first and second drive units.