A foreign object detection device for a wireless power transfer system wirelessly feeding power between a first coil device which has a first coil and is installed on a reference surface and a second coil device which has a second coil and is mounted in a movable object (V), the foreign object detection device comprising: a storage unit configured to store first coil device information including a shape of a first coil facing surface in the first coil facing the second coil device , a size of the first coil facing surface , a height position of the first coil facing surface from the reference surface, and a height position of a first device facing surface in the first coil device facing the second coil device from the reference surface; an information acquiring unit configured to acquire second coil device information including a shape of a second coil facing surface in the second coil facing the first coil device , a size of the second coil facing surface , a height position of the second coil facing surface from the reference surface, and a height position of a second device facing surface in the second coil device facing the first coil device from the reference surface; a region identifying unit configured to identify a magnetic field generation region (X) generated between the first device facing surface and the second device facing surface when feeding power, based on the first coil device information and the second coil device information; a foreign object detection unit configured to detect the presence or absence of a foreign object within the identified magnetic field generation region (X); characterised in that it comprises a position detection unit configured to detect a position of the movable object (V) in a first direction orthogonal to a facing direction of the first coil device and the second coil device , and in that the region identifying unit is configured to identify the magnetic field generation region (X) based on the first coil device information, the second coil device information, and a position of the movable object (V) detected by the position detection unit . a storage unit configured to store first coil device information including a shape of a first coil facing surface in the first coil facing the second coil device , a size of the first coil facing surface , a height position of the first coil facing surface from the reference surface, and a height position of a first device facing surface in the first coil device facing the second coil device from the reference surface; an information acquiring unit configured to acquire second coil device information including a shape of a second coil facing surface in the second coil facing the first coil device , a size of the second coil facing surface , a height position of the second coil facing surface from the reference surface, and a height position of a second device facing surface in the second coil device facing the first coil device from the reference surface; a region identifying unit configured to identify a magnetic field generation region (X) generated between the first device facing surface and the second device facing surface when feeding power, based on the first coil device information and the second coil device information; a foreign object detection unit configured to detect the presence or absence of a foreign object within the identified magnetic field generation region (X); characterised in that it comprises a position detection unit configured to detect a position of the movable object (V) in a first direction orthogonal to a facing direction of the first coil device and the second coil device , and in that the region identifying unit is configured to identify the magnetic field generation region (X) based on the first coil device information, the second coil device information, and a position of the movable object (V) detected by the position detection unit . <EOS>