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 regions of said detection target based on positional relationships between the extracted region candidates and on the probability calculated in said probability calculation step, wherein said probability calculation step further comprises comparing said region candidates with probability density functions that are predetermined for each region of the detection target and stored in a database in order to express the degree each region candidate corresponds to the regions of the detection target.