1. An object detection method comprising the steps of: acquiring an image including a target object; generating a plurality of layer images by one or both of enlarging and reducing the image at a plurality of different scales; detecting a region of at least a part of the target object as a first detected region from at least one of the layer images; estimating a first estimated region based on the first detected region; selecting at least one of the layer images based on the estimated first estimated region; detecting a region of at least a part of the target object in the selected layer image different from the first detected region as a second detected region; estimating a second estimated region based on the second detected region; and integrating the first estimated region and the second estimated region, wherein the first estimated region is estimated based on coordinates and a size of the detected first detected region; and the second estimated region is estimated based on coordinates and a size of the detection result of the second detected region.