1. An object detection method comprising: an image acquisition step of acquiring an image including a target object; a layer image generation step of generating a plurality of layer images by one or both of enlarging and reducing the image at a plurality of different scales; a first detection step of detecting a region of at least apart of the target object as a first detected region from each of the layer images; a selection step of selecting at least one of the layer images based on the detected first detected region and learning data learned in advance; a second detection step of detecting a region of at least a part of the target object in the layer image selected in the selection step as a second detected region; and an integration step of integrating a detection result detected in the first detection step and a detection result detected in the second detection step.