1. An image processing device comprising: a scanning unit that scans an original document; a background-color detecting unit that detects a candidate area of a background color in a chromaticity plane, with respect to the scanned original document; a background-color-area determining unit that determines a background color area based on the candidate area of the background color detected by the background-color detecting unit; and a mask setting unit that generates boundary lines extending to an achromatic-color area from the background color area determined by the background-color-area determining unit, and sets, as a mask, the background color area and an area surrounded by the background color area and the boundary lines.