1. A solid object identifying apparatus for identifying a solid object existing in an image pickup area having an outer surface facing towards a different direction than a predetermined plane, said solid object identifying apparatus comprising: an imaging part configured to receive two polarized lights having a different polarization direction included in light reflected from an object existing in the image pickup area and to capture two polarization images; a luminance calculating part configured to divide each of the two polarization images captured by the imaging part into predetermined pixels, and to calculate a total luminance value in the two polarization images for each of the predetermined pixels; a polarized intensity difference calculating part configured to calculate a polarized intensity difference indicating a ratio of a polarized intensity difference value between the two polarization images to the total luminance value, for each of the predetermined pixels; a relative polarized intensity difference calculating part configured to set the polarized intensity difference calculated by the polarized intensity difference calculating part to be a reference polarized intensity difference in a reference pixel of which a luminance is relatively greater in a pixel corresponding to a reference plane object assumed beforehand to be in a same plane as the predetermined plane, and to calculate a relative difference value between the reference polarized intensity difference and the polarized intensity difference which is calculated by the polarized intensity difference calculating part for a different pixel from the reference pixel; and a solid object identification processing part configured to: conduct a solid object identification process for identifying whether or not an object existing at a place corresponding to each of the predetermined pixels in the image pickup area is the solid object, by using the total luminance value calculated by the luminance calculating part and the relative difference value calculated by the relative polarized intensity difference calculating part, as an identification index value, and conduct a determination process for determining which one of multiple numerical ranges the identification index value belongs to, in which the multiple numerical ranges are defined for each solid objects and a lower luminance portion of the reference plane object existing at a place corresponding to a lower luminance pixel indicating lower luminance in a pixel in which the reference plane object exist, and to conduct a process for identifying an object corresponding to the one of the multiple numerical ranges which is determined in the determination process, so as to conduct the solid object identification process.