A keyless entry system that performs communication between a vehicle-side device provided in a vehicle and a portable device, wherein the vehicle includes a detector that detects a person's action of getting off the vehicle, wherein the portable device has a portable device controller that performs communication of a signal for authentication, and a portable device electrode that makes an electric field induced in a human body by the signal, wherein the vehicle-side device has a vehicle-side electrode that detects the electric field induced in the human body by the portable device electrode, and a vehicle-side controller that has a signal input thereto by an electric field detected by the vehicle-side electrode, and authenticates the signal, and wherein the vehicle-side controller determines that the portable device has been left in the vehicle if the vehicle-side controller has no history of authentication of the portable device when the person's action of getting off a vehicle has been detected by the detector, or if the vehicle-side controller cannot perform authentication of the portable device therewith immediately after the detection, and then makes the device provided in the vehicle perform a predetermined operation.