To provide a key and a key cylinder constituted to lock and unlock a plurality of key cylinders with one key.

The key 4 has key grooves 9a, 9b in a key plate 7. The key groove 9a is formed in groove shape corresponding to a plurality of locking plates 10 arranged on the internal upper face of a keyhole 6 of the key cylinder 1. The key groove 9a is used for locking and unlocking the key cylinder 1, and the other key groove 9b is used for locking and unlocking the other key cylinder. A grip part 8 provided at the base end part of the key plate 7 is provided with marks 15a, 15b put to discriminate the key grooves 9a, 9b. The insert direction of the key 4 can thereby be discriminated. The grip part 8 has a built-in portable unit 21 for transmitting an ID code in response to a request signal from a wireless door locking/unlocking device provided on the vehicle side.