1. An interpolation processing apparatus that performs interpolation processing on an image generated by an image-capturing sensor constituted by two-dimensionally arraying a plurality of lattice points each of which outputs color information corresponding to a first color component and a plurality of vacancies each of which outputs color information corresponding to a second color component or a third color component instead of outputting color information corresponding to the first color component, to determine an interpolation quantity equivalent to color information corresponding to the first color component for a vacancy, comprising: a similarity degree calculation unit that calculates two types of similarity components along each of a plurality of directions, which are a “similarity component obtained by using color information corresponding to the first color component and color information corresponding to the second color component” and a “similarity component obtained by using color information corresponding to the first color component and color information corresponding to the third color component”, for an interpolation target vacancy to undergo said interpolation processing based upon color information corresponding to the first through third color components output from said interpolation target vacancy and lattice points and vacancies near said interpolation target vacancy, and then calculates similarity degrees based upon said two types of similarity components; a similarity judgment unit that makes a judgment with respect to a direction manifesting pronounced similarity for said interpolation target vacancy based upon said similarity degrees along the plurality of directions calculated by said similarity degree calculation unit; and an interpolation quantity calculation unit that calculates said interpolation quantity for said interpolation target vacancy in conformance to results of said judgment made by said similarity judgment unit.