An image forming apparatus has a normal print mode in which input image data for a plurality of pages are processed into images on the same number of pages of paper, and a trial print mode in which the input image data are processed into minified images on fewer pages of the paper for trial output such that a user can check an output result to be obtained in the normal mode beforehand. In the trial print mode, a blank page detector detects a blank page from the input image data, and an image data minifier converts the image data stored in an image data buffer into minified image data and stores the data in a second buffer. Then, a plotting section develops the minified image data read from the second data buffer into a plotting pattern, which is then written to a page buffer. In an output section, the plotting pattern read from the page buffer is processed into a visible image. At this time, an image corresponding to the blank page detected by the blank page detector is not formed.