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