Provided is an electronic key system, comprising a lock-side terminal installed in a lock item which generates an electronic key with which it is possible to carry out at least locking and unlocking of the lock item; and a portable terminal which is capable of carrying out a transmission and receiving of the electronic key with the lock-side terminal which is installed in the lock item and other portable terminals. Upon receiving a request from one portable terminal, the lock-side terminal randomly generates the electronic key, and transmits the randomly generated electronic key to the one portable terminal. It is thus possible, without needing to provide a physical key, card, portable terminal, etc., to provide only an electronic key function to another person (either transferring or copying), to call a person in a distant location to operate on the lock item, etc., which is highly useful.