1. A semiconductor failure analysis apparatus for performing a failure analysis by means of a thermal image of a semiconductor device, comprising: voltage applying means applying a bias voltage to a semiconductor device to serve as an analysis object; imaging means acquiring an image of the semiconductor device; and image processing means performing image processing necessary for a failure analysis of the semiconductor device for an image acquired by the imaging means, wherein the imaging means acquires a plurality of analysis images each including a thermal image in a state where the bias voltage is applied to the semiconductor device and a plurality of background images in a state where the bias voltage is not applied thereto, and the image processing means includes: imaging position calculating means calculating, for each of the plurality of analysis images and the plurality of background images, an imaging position thereof; image classifying means preparing, for the imaging position of each of the plurality of analysis images and the plurality of background images, a region division unit set with reference to a position frequency distribution of the imaging positions, and classifying the plurality of analysis images and the plurality of background images into N image groups (N is an integer not less than 2) according to which of the N regions divided in accordance with the region division unit the imaging position belongs to; and difference image generating means generating a difference image between the analysis image and the background image to be used for a failure analysis, individually for the classified N image groups.