1. An image forming apparatus that executes an image forming process based on print data including an execution command of transparency processes, wherein each transparency process overlaps first image data having a first transparency and second image data having a second transparency, the image forming apparatus comprising: a first storage unit to be used when a transparency process is executed; and a control unit for controlling a first number of transparency processes to be executed when a storage capacity of the first storage unit is insufficient to execute the transparency processes to be executed, and causing a display unit to display a preview image to which a second number of executable transparency processes different from the first number has been executed to determine an image being an object for print output based on an instruction by an operator relating to which transparency process is executed and wherein the control unit calculates the second number of executable transparency processes based on a usable storage capacity of the first storage unit, calculates the first number of transparency processes to be executed based on the print data, and judges whether the storage capacity of the fist storage unit is insufficient or not to execute the transparency processes to be executed by comparing the first number and the second number.