To improve the convenience of a lock device while ensuring the sense of use and the security strength of a conventional lock device.
The lock device is formed of an internal cylinder 20, No.1 to No.3 tumblers 40, and a No.0 tumbler 50. The internal cylinder 20 has a keyhole 21, and carries out unlocking by rotating in an external cylinder 30. The No.1 to No.3 tumblers 40 are arranged in part of slots 22 corresponding to a plurality of key grooves 12 formed at a key 10 to be inserted into the keyhole 21, engaged with the external cylinder 30 when the key 10 is not inserted but move according to the shape of the key grooves 12 by being pressed by the key grooves 12 formed in the key 10 when the key 10 is inserted, to thereby disengage the engagement with the external cylinder 30. The No.0 tumbler 50 is arranged in the slot 22 other than part of the slots 22 where the No.1 to No.3 tumblers 40 are arranged, and is not engaged with the external cylinder 30 irrespective of the shape of the key groove 12 of the key 10 when the key 10 is inserted.