1. An image processing apparatus comprising: an interpolation unit adapted to carry out, after having received color mosaic image data which is generated by an image pickup device having plural-color filters, an interpolation process of interpolating signals of missing colors onto each of pixels of the received color mosaic image data, by using image signals of neighboring pixels of the each of the pixels, and to output the interpolation process-carried out color mosaic image data, wherein said interpolation unit comprises: a direction-by-direction interpolation unit adapted to carry out, after having received the generated color mosaic image data, the interpolation process onto the each of the pixels of the input color mosaic image data in each of a plurality of predetermined directions, and to output a plurality of color image data, each of which corresponds to the each of the plurality of predetermined directions, said direction-by-direction interpolation unit carrying out the interpolation process onto the each of pixels of the received color mosaic image data, by using image signals of neighboring pixels located in a predetermined direction of the each of the pixels; an evaluation unit adapted to calculate, after having received the plurality of color image data output from said direction-by-direction interpolation unit, an evaluation value of each of pixels of the each of the plurality of color image data, by using homogeneity and signal intensity of the each of the pixels, said evaluation unit calculating, based on dispersion of signals of pixels included in a predetermined-sized region, homogeneity of a pixel located in a center of the predetermined-sized region, and calculating, based on saturation of signals of pixels included in the predetermined sized region, signal intensity of the pixel located in the center of the predetermined-sized region; and an image formation unit adapted to generate, after having received the plurality of color image data output from said direction-by-direction interpolation unit and the evaluation value of the each of the pixels of the each of the plurality of color image data calculated by said evaluation unit, signals of the pixels based on the evaluation value of the each of the pixels, said image formation unit being adopted to select, based on the evaluation value of the each of the pixels, one signal from the generated signals of the pixels of the plurality of the color image data onto the each of the pixels, or to generate, based on the evaluation value of the each of the pixels of the plurality of the color image data, one signal of the pixels by combining the signals of the pixels of the plurality of the color image data.