An image forming apparatus and method are disclosed. The image forming apparatus and method comprise a main image forming apparatus having a developing section and capable of being independently driven so that it prints an image onto a paper fed on the basis of input image information; at least one sub-image forming apparatus removably mounted to the main image forming apparatus in sequence, the sub-image forming apparatus having a developing section and modularized to perform printing operation according to a control command. The apparatus and method further comprise a main paper feeding unit connected to the main image forming apparatus and feeding a paper to the main image forming apparatus; at least one sub-paper feeding units removably connected to the main paper feeding unit in sequence for feeding a paper to the sub-image forming apparatus, respectively; and a control section for selectively controlling the operation of respective image forming apparatuses and respective paper feeding units on the basis of input image information in a manner that printing operations are performed either in one image forming apparatus or simultaneously in the respective image forming apparatuses.