To attain operation for folding and opening a telephone set in storage state by mounting the flexible main body of case through a hinge part and fixing it at the side edge of portable telephone through an elastic engage pawl to be engaged from left and right sides.

Concerning a case A, along with the outside face of casing 1 to be inside in case of folding opposite to inside faces 1A and 2A where an operating part 4 or a display part 5 is provided, a main body 11 of plate- shaped case composed of flexible leather or cloth is mounted by the flexible part of hinge part 3. A fixing means 12 for keeping that mounting state is composed of an elastic member such as sheet metal spring, for example, having the engage pawls to be engaged from the left and right sides to a portable telephone set B at its side edge. The main body 11 of case is mounted through a mounting means such as a clip, for example, to the belt of clothing, etc. The main body 11 of case is detached from the belt as needed, the flexible part on the main body 11 of case is bent and the portable telephone set B is operated from folded state to opened state. Thus, the portable telephone set can immediately be turned to the state usable for speaking, etc.