1. A posture state estimation apparatus that estimates a posture state of an object comprising a plurality of parts based on image data of the object, the posture state estimation apparatus comprising: a candidate extraction section that performs, for each of the parts, an extraction of the parts from the image data; an exposed region estimation section that determines an exposed region having an edge, the distance from an edge of a first candidate to the edge of the exposed region being determined from a thickness of a second candidate, the first candidate being for an extracted part of the parts, the second candidate being for an unextracted part of the parts; a complement candidate extraction section that determines a complement candidate region based on the exposed region and a constraint information regarding the parts, and performs an extraction of the second candidate from the image data within the bounds of the complement candidate region, the compliment candidate region being a region where the second candidate is likely located and a portion of which being likely occluded by the first candidate; and an estimation section that estimates the posture state of the object based on the first candidate and the second candidate.