PURPOSE: To improve the job efficiency by preventing an erroneous input when an operator enters a page to which a ship sheet is inserted to insert the slip sheet between desired pages of plural original sheets.

CONSTITUTION: A picture data input section 70 reads a picture of an original 1 in which white paper is inserted in advance to pages of the original to which a ship sheet is inserted, the picture is converted into a digital signal and the digital picture signal subjected to signal processing is stored n a memory 73 and a picture processing CPU 79 discriminates a page whose density is below a prescribed density. While a ship sheet mode selection key of an operation panel 10 is depressed, the ship sheet is inserted to pages to which the white paper whose density is discriminated to be a prescribed density or below is inserted by a command from a paper feed section drive circuit 7.