1. A white balance device for carrying out white balance with regard to an image, comprising: a background pixel extraction unit used to extract plural background pixels of the image; a representative-color-of-background obtaining unit used to obtain representative color of background of the image based on the extracted background pixels; a global magnification ratio calculation unit used to determine a global magnification ratio of the background pixels based on predetermined expected background color and the representative color of background; a color pre-adjustment unit used to carry out pre-adjustment with regard to original color of each of the background pixels by using the global magnification ratio so as to obtain pre-adjusted color of the corresponding background pixel; an accurate magnification-ratio-of-background-pixel calculation unit used to, for each of the background pixels, determine expected final background color of the corresponding background pixel based on the pre-adjusted color of the corresponding background pixel, and then determine an accurate magnification ratio of the corresponding background pixel based on the expected final background color and the original color of the corresponding background pixel; and an accurate color adjustment unit used to, for each of the background pixels, carry out adjustment with regard to the original color of the corresponding background pixel by using the accurate magnification ratio of the corresponding background pixel.