A vehicular electronic key system includes a vehicle having a key cylinder and a portable device. The portable devise includes a key body and a mechanical key. The key body is communicatable with the vehicle. The mechanical key is attachable to and detachable from the key body and mechanically rotates the key cylinder whereby the communication between the vehicle and the portable device allows the vehicle to execute predetermined functions. The key body includes a key detector configured to detect an attachment state and a detachment state of the mechanical key to and from the key body so that the key body transitions to a valet mode to limit some of the predetermined functions executable by the vehicle in accordance with the communication between the vehicle and the portable device while the key detector detects the detachment state of the mechanical key from the key body.