An image processing apparatus includes: a receiving unit for receiving an additional image data to be overlaid on an output image, a modification unit for modifying the additional image data received by the receiving unit so as to shorten the processing time of converting the additional image into a binary image compared with before it modified, a storage unit for storing the additional image data modified by the modification unit, a converting unit for converting the modified additional image data stored in the storage unit into a binary image data when instruction of overlaying the additional image on the output image has been done, and a drawing unit for drawing the binarized additional image data converted by the converting unit on the output image.