Polyester composition comprising at least one polyester; and an epoxy system comprising diphenolic epoxy condensation polymer, and at least one epoxy compound comprising at least two epoxy groups per molecule of the epoxy compound. The sealing element is preferably prestressed, with the gas pressure supporting the sealing effect. The detecting section detects types of recording materials to be used in units of the plural print jobs on the basis of the acquired print information. The present invention realizes free combination and disassembly of key groups by a method recognizing the free-combined key groups.