rod 41 the first link rod 41 is always pushed by this spring 44 in such a way as to outwardly be protruded this pushing force is transmitted to the second link rod 42 through the arms 52 a and 52 b of the rotation lever 43 so that the second link rod 42 is elastically pushed in such a way as to outwardly project therefrom the spring 44 constitutes an elastic member according to the invention incidentally various kinds of spring members or rubber can be used as the elastic member the paired left and right lock pieces 45 of the lock device 40 are inserted into lock piece insertion holes 38 which are formed in side walls of the lid body 31 thereby to be projected from both sides of the lid 30 at that time each of the engaging claws 51 of the guide shafts 50 engages with the peripheral edge of the associated guide hole 48 to thereby restrict a maximum amount of projection thereof further the receiving portion 55 of the first link rod 41 is projected from a receiving portion insertion hole 39 formed in an upper portion of the right side face of the lid body 31 as shown in fig6 this receiving portion insertion hole 39 has a cutout 39 a formed toward the back surface of the lid 30 a pusher 74 of an operating element 70 to be described later can be passed through this cutout 38 a.