A plug-in unit-mounting structure which makes it possible to efficiently accommodate plug-in units without being dependent on the structure of a shelf. The plug-in unit-mounting structure comprises a shelf that has connectors disposed on a back wiring board, for being fitted to the plug-in units, and at least one mounting frame that is mounted in the shelf, for each accommodating an associated one of the plug-in units, and enabling the plug-in unit to be inserted in and removed from an associated one of the connectors, the mounting frame being provided with a guide rail which enables a position of disposition of the plug-in unit to be changed according to a size of the plug-in unit. This makes it possible to efficiently accommodate plug-in units without being dependent on the structure of a shelf.