Storage box for storing stacked recording mediums comprises a two-part structure consisting of in-apparatus mounting portion and out-apparatus mounting portion which are coupled via a rotational support mechanism. In-apparatus mounting portion and out-apparatus mounting portion of storage box are kept integrally coupled via lock mechanisms for locking in-apparatus mounting portion and out-apparatus mounting portion of storage box in normal use. On the other hand, when an improper handling is performed such that the whole image forming apparatus is lifted up by gripping the out-apparatus mounting portion of paper feed cassette, lock mechanisms are released to pivot out-apparatus mounting portion thereby to avoid the improper handling so that paper feed cassette which is exposed outside from the image forming apparatus is prevented from damages and the risk of the whole apparatus falling is avoided beforehand.