To provide a visible card key for each user of a hotel on which a room number can be easily written or erased thereby eliminating the user to memorize the room number and a person working at the front office becomes free from troublesome work such as handing over a paper envelope, a memo or a card clearly marked with a room number to each user.

A visually indicating means is provided on the whole or part of the surface of the card key for a card key type electronic lock capable of locking and unlocking a door with the card key.