1. A target object distance measuring apparatus including a human body detector for detecting a human body based on an image acquired by an image capturing device, the target object distance measuring apparatus comprising: a human body candidate area extractor for extracting a human body candidate area from the image; a reference feature extractor for extracting, as a reference feature from the extracted human body candidate area, a predetermined one of human features including a total height, a torso, an arm, a leg, and a head; a comparison feature extractor for extracting, as a comparison feature from the extracted human body candidate area, one of the human features including the total height, the torso, the arm, the leg, and the head, except for the reference feature; a size-in-real-space estimator for estimating the size in real space of a human body candidate in the image, based on a ratio of the size of the reference feature and the size of the comparison feature; and a distance calculator for calculating the distance from the image capturing device to the human body candidate in the real space, based on the estimated size in the real space of the human body candidate and the size of the human body candidate in the image.