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 tumbler unit that is mounted between the housing and the cylindrical core to permit or prevent the relative rotation between the housing and the cylindrical core and that has a first pin extending 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 first pin, so as to switch the tumbler unit to permit the relative rotation between the cylindrical core and the housing.