Abstract: This invention relates to the design and construction of hinge units for joining a case and its corresponding lid, which comprises: a pair of bend units with a receptacle body in between for keeping a spring integrally connected to a side wall of a case; a guide member slidably inserted in the hollow space of the receptacle body and supported by the spring; a joint plate having a through opening running transversely through the lower portion thereof to be joined with the bend units by means of an inserted pin; and a socket integrally connected to a side wall of a lid for receiving a portion of the joint plate therein.