Abstract: In casing-in bookbinding, in which a book sheet block of book sheets is wrapped with a cover, an input of pieces of information about the length of a short side of the book sheets, the length of a long side of the cover, and the thickness of a spine cover of the cover is received. Judgment is made, based on the received information, whether the length of the long side of the cover is less than a value obtained by adding the thickness of the spine cover to twice the length of the short side of the book sheets. If the length of the long side of the cover is less than the obtained value, an alert is issued to urge the user to change at least one of the pieces of information.