Abstract: A locking device for manhole inner covers is disclosed. The locking device is operated only by an authorized key thus almost completely protecting the underground arrangements such as pipes, drains or wires from unauthorized persons, who intend to reach the arrangements with the object of damaging or breaking the arrangements. The locking device is cased by a longitudinal channel type housing. Two reciprocating units are arranged in the housing so that the arrangement of the units inside the housing is point-symmetric about the center of the housing. Each unit has a plurality of regularly-spaced gear holes, which selectively engage with the asterisk part of a key. A locking bolt is coupled to each reciprocating unit and fully projects or retracts from or into the housing. A keyhole panel is movably seated on a central member so that the keyhole panel can move in a vertical direction. The keyhole panel has an asterisk keyhole.