1. An image forming apparatus comprising: a selector for allowing a user to select whether to perform monochrome or multi-color image processing operation on image data obtained by scanning an original image; an automatic image forming mode setting section for defining image forming conditions including such parameters as copying density which are automatically set in automatic image forming mode; and a controller for outputting, depending on whether the user has selected the monochrome or multi-color image processing operation through the selector, a control signal indicating whether or not to prescan the original image for distinguishing the type of original when the automatic image forming mode is selected to use the image forming conditions defined in the automatic image forming mode setting section.