to the locking piece 12 so as to drive the locking piece 12 to move together in order to help the user to push the push button 14 easily the upper surface of the button body 140 is formed with a convex surface 1400 when the user intends to press the push button 14 the user can press the convex surface 1400 of the button body 140 with his her finger the locking piece 12 includes a pushing portion 120 connected to the push button 14 and two stoppers 122 connected to both sides of the pushing portion 120 the locking piece 12 is connected to elastic pieces which are for example springs 18 in the present embodiment the upper and lower surfaces of the pushing portion 120 protrude from the stopper 122 the pushing portion 120 is formed with a button locking trough 1200 for engaging with the button hooks 142 two button locking troughs 1200 are provided to correspond to the two push buttons 120 respectively on the upper and lower surfaces of the pushing portion 120 the button hooks 142 are received in the button locking trough 1200 and fixed therein one end surface of the pushing portion 120 facing the second fastener 2 is located at a level lower than that of the stopper 122 the stoppers 122 are symmetrically provided on both sides of the pushing portion 120 the upper surface or the lower surface of the stopper 122 is formed with a stopping strip 125 a mounting groove 127 is formed between the stopping strip 125 and the pushing portion 120 the upper surface or the lower.