To provide an electronic key system, an on-board unit, and a portable device, by which a user is allowed to board or start engine when wireless communication between the portable device and the on-board unit is unavailable.

An on-board unit transmits a response request signal through the radio wave in the first frequency band, estimates presence/absence of jamming to the radio wave in the second frequency band based on the reception state of the response signal transmitted from the portable device with the radio wave in the second frequency band, identifies a transmission timing of the response request signal upon generation of jamming, and transmits a notification instruction signal that contains the transmission timing for instructing notification of operation guide information that guides with respect to the operation of the portable device corresponding to the transmission timing. The portable device notifies the operation guide information based on the transmission timing contained in the received notification instruction signal upon reception of the notification instruction signal from a receiver of the first portable device.