1. A thermal image sensor that scans thermo piles to acquire temperature data of pixels of the thermo piles that are arranged in a predetermined direction, on a per step basis in a direction vertical to said predetermined direction, and that synthesizes a one-dimensional thermal image acquired in each step to acquire said thermal image in two dimensions, said thermal image sensor comprising: a controller coupled to a memory and configured to determine positions of detected significant pixels which are selected, as significant pixels, in an immediately preceding step from among all pixels of the thermo piles; scan to acquire said temperature data of said significant pixels; generate weight values for all said pixels on a basis of both a relative position of each pixel relative to each of said significant pixels and said temperature data; and select said detected significant pixels to be used in a next step in descending order of said weight values from among all said pixels.