To prevent a porous sheet from being wasted by obtaining the size of an output image on the basis of input data and selecting an output medium corresponding to the size when an image consisting of an ink permeable part and an ink impermeable part is formed on the surface of the porous sheet used as a stamp.

During the use of a stamping plate making apparatus, when a plate making start key is operated, CPU calculates the size necessary for plate making and judges whether the calculated length can be received in an L-print of largest plate making size or not, and when judged YES, the next key input is to be waited. Whether a character line for plate making can be received in an S-print or not is judged on the basis of the length necessary for plate making, and when judged YES, a message is indicated on a display 102 to set a porous sheet having the S-print. When the setting of a printing plate holder is confirmed from the output of a micro-switch 93, thermal heads 65, etc., are driven for plate making.