The invention provides a keyless entry apparatus capable of accurately determining whether a portable apparatus is positioned inside or outside an interface between an inner surface and an outer surface of a vehicle even when a region which is affected by crosstalk and/or in which saturation occurs exists outside or inside the vehicle. When an in-vehicle control unit 12 detects that a portable apparatus 3 is positioned in a region which is affected by crosstalk and/or in which saturation occurs, in the vicinities of some of a plurality of transmitting antennas 15, on the basis of intensity data from some of the plurality of transmitting antennas that is detected by a portable apparatus control unit 22, the in-vehicle control unit 12 determines whether intensity data from the transmitting antenna that is detected by the portable apparatus is included in a data group composed of a plurality of intensity data that are transmitted from the transmitting antennas and then received by the portable apparatus when the portable apparatus is positioned along the inside of an interface between an inner surface and an outer surface of the vehicle in the region. When it is determined that the intensity data is included in the data group, the in-vehicle control unit determines that the portable apparatus is positioned inside the vehicle. When it is determined that the intensity data is not included in the data group, the in-vehicle control unit determines that the portable apparatus is positioned outside the vehicle.