A guide groove (31) has two protrusions (4) on a side face (311), and a slider (32) has three concave grooves (5), into which the two protrusions (4) are capable of being fitted, on a sidewall (324). When the slider (32) is located at a lock position, the two protrusions (4) are fitted into two of the three concave grooves (5) on one end side of the guide groove (31) to restrict movement of the slider (32). When the slider (32) is located at an unlock position, the two protrusions (4) are fitted into two of the three concave grooves (5) on the other end side of the guide groove (31) to restrict movement of the slider (32).