1. An imaging device comprising: an imaging element constituted of a plurality of pixels having different photoelectric conversion characteristics with respect to an inflection point; and assuming that one of the different photoelectric conversion characteristics is a reference photoelectric conversion characteristic, and the other thereof is a non-reference photoelectric conversion characteristic, a characteristic converting section which converts a pixel value obtained from the non-reference photoelectric conversion characteristic into a pixel value obtained from the reference photoelectric conversion characteristic, wherein the characteristic converting section converts the pixel value obtained from the non-reference photoelectric conversion characteristic into the pixel value obtained from the reference photoelectric conversion characteristic by using predetermined conversion information with respect to each of the pixels to eliminate variation of the reference photoelectric conversion characteristic of each of the pixels after the conversion, the variation of the reference photoelectric conversion characteristic resulting from variation of the non-reference photoelectric conversion characteristic of each of the pixels, wherein the characteristic converting section calculates an approximation equation expressing the non-reference photoelectric conversion characteristic of each of the pixels by using the conversion information to convert the pixel value obtained from the non-reference photoelectric conversion characteristic into the pixel value obtained from the reference photoelectric conversion characteristic by using the calculated approximation equation, wherein the approximation equation is obtained by approximating the non-reference photoelectric conversion characteristic with a quadratic function, and wherein the conversion information is coefficients of the quadratic function.