An electronic key system which checks, using two-way communication, a key code stored in an electronic key against a key code stored in a lock device that locks an entrance to a predetermined space, and allows an unlocking operation of the lock device if the key code stored in the electronic key matches the key code stored in the lock device, is such that the key code includes an identification code unique to the electronic key and a property code indicative of a property of the electronic key (a master key or an additional key). The lock device erases a key code that includes a property code indicating that the key is an additional key, from among the key codes stored in the lock device, when a predetermined operation (including an operation using the master key) is performed.