A first ratchet claw and a second ratchet claw are formed on a lock member which is provided in a reciprocating manner with respect to a guide member, wherein a plurality of first and second ratchet recesses for being fitted to the first and second ratchet claw and the first and second ratchet claw are provided on the guide member along a sliding direction of the guide member, and wherein when a lever member which is slidably provided on the guide member is in the unlocked position, the first and second ratchet recesses are continuously formed along a sliding direction of the guide member arranged in an relationship in which at least one of the first and second ratchet recess does not face the first ratchet claw in the case of the first ratchet recess or the second ratchet claw in the case of the second ratchet recess.