To provide a card key and a key cylinder having improved reliability with simple structure.

The card key 3 is inserted into the key cylinder provided in a door or the like of a vehicle, and if the card key 3 is a regular key, it can lock or unlock the door or the like. The card key 3 has a resin-made card-shaped key body 30, and magnets 31A-31I are buried in the key body 30 to form a predetermined code pattern. In addition, at least a surface of the key body 30 is covered with an opaque material to prevent a person from recognizing the magnets 31A-31I visually from the outside.