A color image forming apparatus, comprising: a plurality of drum-type photoconductors for forming an image in a different color on each peripheral surface and the photoconductors having at least two different diameters; a plurality of driving sections for driving each photoconductor at a driving speed in accordance with the diameter so that each photoconductor rotates at a predetermined peripheral speed; a correction signal output section (43, 51) for outputting a speed correction signal to correct a periodic pitch fluctuation included in each formed image; and a drive control section (40, 53) for controlling the driving section to correct the driving speed of each photoconductor by the speed correction signal; wherein the speed correction signal is a signal having the same cycle as a rotational cycle of each photoconductor; further comprising: a registration image forming section for forming a registration image comprising a plurality of patterns on each photoconductor; a measurement section for measuring a position of each pattern of the formed registration image; and a fluctuation component calculation section for calculating an amplitude and a phase of a pitch fluctuation component corresponding to the rotational cycle of the photoconductor based on a measurement result of each pattern; wherein the correction signal output section (43, 51) includes a correction signal generating section for generating the speed correction signal for every kind of the diameters based on the calculated amplitude and phase; wherein the speed correction signal is a common signal of the photoconductors having mutually the same diameter, and wherein the correction signal generating section is configured to calculate an average of a maximum amplitude and a minimum amplitude of the amplitude of the pitch fluctuation of each photoconductor to which the speed correction signal is applied, and is configured to generate the speed correction signal by using the calculated amplitude. a plurality of drum-type photoconductors for forming an image in a different color on each peripheral surface and the photoconductors having at least two different diameters; a plurality of driving sections for driving each photoconductor at a driving speed in accordance with the diameter so that each photoconductor rotates at a predetermined peripheral speed; a correction signal output section (43, 51) for outputting a speed correction signal to correct a periodic pitch fluctuation included in each formed image; and a drive control section (40, 53) for controlling the driving section to correct the driving speed of each photoconductor by the speed correction signal; wherein the speed correction signal is a signal having the same cycle as a rotational cycle of each photoconductor; a registration image forming section for forming a registration image comprising a plurality of patterns on each photoconductor; a measurement section for measuring a position of each pattern of the formed registration image; and a fluctuation component calculation section for calculating an amplitude and a phase of a pitch fluctuation component corresponding to the rotational cycle of the photoconductor based on a measurement result of each pattern; wherein the correction signal output section (43, 51) includes a correction signal generating section for generating the speed correction signal for every kind of the diameters based on the calculated amplitude and phase; wherein the speed correction signal is a common signal of the photoconductors having mutually the same diameter, and wherein the correction signal generating section is configured to calculate an average of a maximum amplitude and a minimum amplitude of the amplitude of the pitch fluctuation of each photoconductor to which the speed correction signal is applied, and is configured to generate the speed correction signal by using the calculated amplitude.