1. An image processing device comprising: an image acquisition unit configured to acquire a mosaic image, wherein the mosaic image includes pixels of a first color that contributes most to acquisition of a luminance signal among pixels of three primary colors, and pixels of second colors composed of two or more colors other than the first color, and wherein a first color image corresponding to the pixels of the first color and a second color image corresponding to the pixels of the second colors have respective different frequency ranges that are reproducible; a region extraction unit configured to extract a region containing a spatial frequency at which the second color image is not reproducible while the first color image is reproducible, in an entire image area corresponding to the mosaic image acquired by the image acquisition unit; a demosaic processing unit configured to determine pixel values of other colors at a position of each pixel in the mosaic image acquired by the image acquisition unit; and a chroma reduction unit configured to reduce chroma of an image corresponding to the region extracted by the region extraction unit within an image processed by the demosaic processing unit, wherein the region extraction unit comprises: an acquisition unit configured to move pixels or an image of a kernel size to carry out sequential acquisition from the pixels of the first color in the mosaic image acquired by the image acquisition unit, the first color images among the color images of the three primary colors processed by the demosaic processing unit, or a luminance image created from the color images of the three primary colors; a first calculation unit configured to calculate a mean value of pixels on a line in a direction perpendicular to a direction in which a non-reproducible spatial frequency exists, in the acquired pixels or image of the kernel size; a second calculation unit configured to detect a spatial frequency having the non-reproducible spatial frequency and a phase, based on the calculated mean values of pixels, so as to calculate an amplitude of the mean values of pixels at a time of detection; and a detection unit configured to detect the non-reproducible spatial frequency, based on a magnitude of the calculated amplitude.