An image forming apparatus including: a first and a second photoconductor groups constituted of one or more photoconductors respectively; a first and a second drive control sections for controlling the drive of the first and the second photoconductor groups respectively to rotate the photoconductors thereof, wherein the rotational phases of the first photoconductor group and the second photoconductor group are adjusted to be matched therebetween; and the first and the second drive control sections control so that predetermined profile of a target speed is applied to the first and second photoconductor groups wherein, in the target-speed profile, the first photoconductor group starts rotating after a elapse of a predetermined startup delay time from the second photoconductor group starts rotating, and both groups end at a same final speed predetermined for full-color image formation, wherein the startup delay time is predetermined based on measurements of times needed for each of the first and the second photoconductor groups to reach a predetermined speed from starting the rotation with the target-speed profile being applied thereto.