1. An imaging device comprising: an imaging unit including a color filter in which filter elements of three primary colors of red, green and blue are arranged with a predetermined array in a plane thereof, the imaging unit being configured to generate a first frame of an image signal by capturing an object in a state where a first infrared light having a first wavelength assigned to the red is projected, generate a second frame of an image signal by capturing the object in a state where a second infrared light having a second wavelength assigned to the green is projected and generate a third frame of an image signal by capturing the object in a state where a third infrared light having a third wavelength assigned to the blue is projected; a same-position pixel adding unit configured to add pixel data for the red located at common pixel positions in the respective first to third frames to each other, add pixel data for the green located at common pixel positions in the respective first to third frames to each other and add pixel data for the blue located at common pixel positions in the respective first to third frames to each other so as to generate first added pixel data for the red, second added pixel data for the green and third added pixel data for the blue; a synthesizing unit configured to arrange pixel data for the three primary colors including the first added pixel data, the second added pixel data and the third added pixel data generated by the same-position pixel adding unit in such a manner as to have an array identical to the array of the filter elements in the color filter so as to generate a synthesized image signal synthesized in one frame; and a demosaicing unit configured to execute, based on the frame of the synthesized image signal generated by the synthesizing unit, demosaicing for generating a frame of the red in such a manner as to interpolate pixel data for the red in a pixel position where the pixel data for the red is not present, generating a frame of the green in such a manner as to interpolate pixel data for the green in a pixel position where the pixel data for the green is not present and generating a frame of the blue in such a manner as to interpolate pixel data for the blue in a pixel position where the pixel data for the blue is not present, so as to generate frames of the three primary colors.