A locking device includes a lock and a key. The lock includes a housing, a cylindrical core inserted into and rotatable relative to the housing, and a limiting member that is mounted movably between the housing and the cylindrical core to permit or prevent the relative rotation between the housing and the cylindrical core and that extends into a helical keyway formed in the cylindrical core. The key has a helical blade capable of being inserted into the helical keyway to access the limiting member, so as to move the limiting member to permit the relative rotation between the cylindrical core and the housing.