1. A head detecting method using: a preliminary head detection model acquired with images each containing at least a part of a head in a defined image region defined preliminarily as positive examples and with images each not containing a head in the defined image region as negative examples; and a definitive head detection model acquired with images each containing a head in a state where it matches preliminarily defined position and size in the defined image region as positive examples and with images each containing a head in a state where it does not match at least one of the preliminarily defined position and size in the defined image region as negative examples, the method comprising: an image acquiring step of acquiring an image to be detected; a preliminary head detecting step of cutting out the defined image region of the image to be detected as an image patch and detecting head images from the image to be detected by referring to the preliminary head detection model; and a definitive head detecting step of detecting a definitive head image from plural head images acquired in the preliminary head detecting step by referring to the definitive head detection model.