To provide a system for detecting folding of a booklet medium, the system including a scanner and a printing unit that can reliably detect that a page of a booklet medium is folded.

The system for detecting the booklet medium includes: the scanner unit 12 for reading a page mark on a printing surface of the booklet medium; a page mark validity determination unit 6 for determining whether the read page mark is valid or not; and a medium returning unit 13 for returning the booklet medium when it is determined that the read page mark is invalid.