To provide an electronic key system capable of reducing the current consumption of a built-in battery.

A reception unit 12 constituting a portable instrument 1 discriminates whether or not a signal received by a portable instrument side reception antenna 10 is noise, and determines the voltage level of the noise when it is discriminated that the signal is noise. A control unit 13 constituting the portable instrument 1 turns on and turns off an LED 19 by the pattern according to the voltage level of the determined noise. Thus, a user can check the noise reception state of the portable instrument 1. When the user confirms that the portable instrument 1 receives more noise, the user can prevent the portable instrument 1 from being stored at a noise environment, for example, a storage place of the portable instrument is changed. Thus, the current consumption of a built-in battery 15 can be reduced.

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