The invention provides a double-side polishing method including first polishing at a high polishing rate, second polishing at a low polishing rate, dividing a straight line extending between the outermost circumferences of the wafer through the center into prescribed sections, and optically measuring a cross-sectional shape of the sections; applying a weight predetermined for each section to the cross-sectional shape to quantify flatness of each section; and determining polishing conditions of the first and second polishing in subsequent polishing on a basis of the quantified flatness, wherein a beam diameter of a measurement apparatus used to measure the cross-sectional shape of outermost sections is smaller than that used to measure the cross-sectional shape of the other section. The method can measure the shape of the wafer up to its outermost circumference with high precision without reducing productivity, and improve the flatness of the entire wafer including its outermost circumference.