To provide an electronic key system capable of enhancing crime prevention by preventing a person from performing processing such that a door is unlocked by means of an electronic key, for example, left indoors, against the user's intention; and to provide a building.

The building includes a locking and unlocking device 17 which locks and unlocks an entrance door, and outdoor-side and indoor-side controllers 21 and 31 which control locking/unlocking performed by the locking and unlocking device 17. The building also includes an outdoor-side communication section 22 capable of communicating with the electronic key 40 in such a manner that a predetermined area astride indoor and outdoor sides is set as a communication area, and an indoor-side communication section 32 capable of communicating with the electronic key 40 in such a manner that a predetermined indoor area is set as a communication area; and the communication areas are set to overlap each other. When an ID code signal from the electronic key 40 is received by the outdoor-side communication section 22, the door is locked and unlocked by the locking and unlocking device 17. On the other hand, when the ID code signal is received by the indoor-side communication section 32, the door is not locked and unlocked by the locking and unlocking device 17.

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