The index finger of the right hand of the user is applied to a front surface of a fixed knob, the thumb of the right hand is applied to a rear surface of a movable knob, and the movable knob is slid by the thumb forwardly toward the fixed knob. When the movable knob is slid, a locking member moves forwardly in unison with the movable knob away from engaging surfaces of walls, whereupon opposite ends of the locking member disengage from the engaging surfaces of the walls. A buckle is now swingable out of a cavity about a pivot shaft. Then, the user grips the fixed knob and the movable knob and lifts the free end of the buckle to erect the buckle, and thereafter opens a front case member and a rear case member away from each other.