To determine whether or not execution of predetermined post-processing is instructed to be executable rightly or not after forming a predetermined image over a paper sheet whereon an image is formed beforehand.

A sheet image capturing section 500 captures first image data outputted from a printing sheet feeding section 200 or a scanner 210. A print data converting section 502 acquires print data and converts the print data into second image data and instruction information. A processing section 504 receives the first image data outputted by the sheet image capturing section 500, the second image data outputted by the print data converting section 502 and third image data stored in a post-processing image storage section 506, forms a predetermined image over a sheet, on which an image is formed beforehand, in accordance with the instruction information outputted by the print data converting section 502, and then outputs image data of an image indicating a result of executing the predetermined post-processing to a UI device 22.