1. A detection method for detecting a detection target formed of a plurality of regions, from an image, the method comprising: an extraction step of extracting region candidates which become candidates of said plurality of regions from said image; a probability calculation step of calculating a probability that each of the extracted region candidates is at least one of said regions; and a selection step of selecting a combination of the region candidates having a high probability as being said detection target based on the probability calculated in said probability calculation step.