1. An interpolation image generation apparatus comprising: an image acquirer which acquires photographed images in which an object is photographed from viewpoints; an actual pixel extractor which (i) extracts, for each of actual pixels composing the photographed images, another actual pixel which is estimated to correspond to a same portion of the object, from pixels in a photographed image other than a photographed image which contains the actual pixel, as a corresponding actual pixel of the actual pixel, and (ii) calculates, for each of the actual pixels, reliability with the extracted corresponding actual pixel thereof; a model definer which defines a model of an interpolation image corresponding to a new viewpoint which is not included in the viewpoints; an interpolation pixel extractor which extracts, for each of the actual pixels, interpolation pixel candidates which are estimated to correspond to a portion of the object to which the actual pixel corresponds, from interpolation pixels composing the model of the interpolation image, based on (i) a position of a viewpoint from where the actual pixel was photographed, (ii) a position of a viewpoint from where the corresponding actual pixel of the actual pixel was photographed and (iii) a position of the new viewpoint; a determiner which determines whether each of the interpolation pixel candidates corresponds to the actual pixel based on (i) the reliability of the actual pixel or (ii) reliability of an actual pixel corresponding to an interpolation pixel adjacent to the interpolation pixel candidate; and an interpolation image generator which updates a pixel value of an interpolation pixel which was determined to be correspond to the actual pixel by the determiner, based on a pixel value of the actual pixel thereby to generate the interpolation image.