A remote keyless entry system includes an in-vehicle device including an in-vehicle device controller, an in-vehicle device transmitter that transmits an in-vehicle device signal, and an in-vehicle device receiver that receives a portable device signal, and a portable device including a portable device controller, an acceleration sensor, a portable device receiver, and a portable device transmitter, wherein the portable device controller calculates a distance from the in-vehicle device to the portable device based on the in-vehicle device signal, calculates a first travel distance based on acceleration, and causes the portable device transmitter to transmit the portable device signal including at least the distance, the in-vehicle device controller receives the portable device signal multiple times and calculates a second travel distance based on distances included in received portable device signals, and whether a relay attack is performed is determined based on the first travel distance and the second travel distance.