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 left indoors, in a vehicle interior, etc., against a user's intention.

An entrance door of a building is provided with a locking and unlocking device 21 which locks and unlocks the entrance door on the basis of authentication of an ID code transmitted from a key-side communication portion 32 of the electronic key 30. The electronic key 30 is equipped with an vibration sensor 35 for detecting that the electronic key 30 is vibrated. When the vibrations are not detected for a predetermined period of time by the vibration sensor 35, a key controller 31 of the electronic key 30 determines that the electronic key 30 is not shifted, and turns off a power source of the key-side communication portion 32. Thus, since the transmission of the ID code signal from the key-side communication portion 32 is stopped, the entrance door can be prevented from being unlocked against the user's intention by means of the electronic key 30 left in the building.

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