PURPOSE: To make a cover reliably double openable in a simple structure by forming an engaging member by means of a connecting element connected to one of a body and a cover, an engaging element to be engaged with the other one, and a pivotally supporting member for rotatably supporting both elements.

CONSTITUTION: The both end upper parts of a body 1 and the both end lower parts of a cover 2 are connected by engaging members 6 so as to be freely released, when one side of the cover 2 is opened, the engaging member 6 is released and turned by taking the other engaging member 6 as a fulcrum. Each engaging member 6 is provided with an attaching fitting 13 attached on the lower inner surface of the cover 2, and a bar receiver 15 whose one end lower part is rotatably and pivotally supported to the lower end of the attaching fitting 13 through a pivotally supporting pin 14, and in which an inserting bar 8 of a locking means is to be inserted to the other end lower part inner surface, and an engaging fitting 17 whose intermediate part is fit-supported on the upper edge 16 is provided. When the right side is opened, a key cylinder 9 of the right engaging member 6 is turned, and the inserting bar 8 is drawn from the bar receiver 15 by sliding a rod 7.