An electronic key system includes an onboard apparatus and a portable apparatus. The electronic key system establishes wireless communication with a secret key common between the onboard apparatus and the portable apparatus, and authenticates the portable apparatus registered as the portable apparatus of an authorized user. The portable apparatus includes a portable apparatus code transmitter that transmits a portable apparatus code. The onboard apparatus includes an onboard code transmitter that transmits an onboard code. The onboard apparatus further includes a portable apparatus code receiver, and an onboard-side key generation portion. The portable apparatus further includes an onboard code receiver, and a portable-apparatus-side key generation portion.