An image forming apparatus includes a first submodule used for image formation; a first phase detector detecting a phase of the first submodule; a second submodule used for image formation with the first submodule; a second phase detector detecting a phase of the second submodule; a density detector detecting density of an image formed by the first and second submodules; a correction setting section setting a first and second parameters to correct density non-uniformity in a slow-scan direction caused by the first and second submodules, respectively, based on the detected image density data; an output setting section deriving a first correction value for the phase of the first submodule from the first parameter and a second correction value for the phase of the second submodule from the second parameter, and outputting a correction value generated by merging the first and second correction values; and an imaging condition changing section changing imaging conditions in accordance with the correction value.