1. A people counting device comprising: a person presence region extractor which extracts a region in which a person is present by performing person detection in relation to an input image; a person presence state estimator which estimates a person presence state from an extraction result of the person presence region extractor, the person presence state estimator estimates a direction in which persons are overlapping based upon the extraction result of the person presence region extractor; a portion detector selector which selects a type of portion detector to be used for people count measurement based on an estimation result of the person presence state estimator, and outputs portion detector information; a portion detection processor which performs a portion detection process for detecting a portion of a person based on the portion detector information output from the portion detector selector; and a people counter which obtains a number of people within the input image from a portion detection process result of the portion detection processor.