1. A person tracking method comprising: a correspondence calculation step of calculating a statistically average correspondence between a size of a head of a person appearing on an image taken by a camera and a position representing a height of the head on the taken image, the camera being disposed at a position looking down a measured space and generates image data representing a moving image of a predetermined frame rate by taking the moving image of the measured space; a head detecting step of detecting, based on each piece of image data representing each of a plurality of measured frame images among a plurality of frame images obtained through image-taking by the camera, a position and a size of a head on each of the measured frame images; a movement-feature-quantity calculation step of calculating, based on positions and sizes of heads on a plurality of past measured frame images detected in the head detecting step and the correspondence calculated in the correspondence calculation step, a movement feature quantity that represents a possibility that a head on a current measured frame image detected in the head detecting step is a head that belongs to the same person as a person having the heads on the past measured frame images; and a same-person determination step of determining, by using the movement feature quantity calculated in the movement-feature-quantity calculation step, whether the head on the current measured frame image is the head that belongs to the same person as the person having the heads on the past measured frame images.