A wireless communication system includes an in-vehicle device mounted on a vehicle; and a portable device that can be carried by a user. The in-vehicle device includes a first sending unit configured to intermittently and wirelessly send an identification code of the vehicle or the in-vehicle device; and a detection unit configured to detect a presence of the portable device by receiving a response signal wirelessly sent from the portable device. The portable device includes a receiving antenna configured to receive the identification code; a first verification unit configured to verify the identification code, received by the receiving antenna, with a code registered in the portable device; and a sending control unit configured to wirelessly send the response signal if the verification by the first verification unit is successful and not to send the response signal if the verification by the first verification unit is not successful.