A keyless entry device can accurately judge or determine whether a portable device exists inside or outside a vehicle. A vehicle-side controller includes a memory that stores an inside data group having a plurality of intensity information of a request signal transmitted from the plurality of transmitting antennas when the portable device is located along an inside of a vehicle. Also included is an outside data group having a plurality of intensity information of a request signal transmitted from the plurality of transmitting antennas when the portable device is located along an outside of the vehicle. The vehicle-side controller judges an area where the portable device exists from the intensity information of the request signal received from the portable device. The controller instantly judges the inside or the outside of the vehicle, when the portable device exists in an area apart from a boundary. The controller also judges which data group of the inside and outside data groups stored in the memory approximates the intensity information when the portable device exists in the vicinity of the boundary.