A PIU plug-in/plug-off mechanism for an electronic apparatus, which is capable of efficiently applying a force required for a plugging-in/off operation of connectors to a printed board by pressing a portion of the printed board close to a backplane. An operating handle of a card lever is turned counterclockwise to thereby bring a push pin into abutment with a rear edge of an open recess. When the card lever is further turned in this state, a pressing force for moving a plug-in unit in the insertion direction acts on an engaging member via an arm. This starts fitting the printed board into a plug-in connector without causing warping of the printed board.