A method for measuring flatness of a sheet, comprising: a step of acquiring a pattern image by projecting a light and dark pattern composed of light portions and dark portions onto a surface of the sheet that is traveling between adjacent rolling stands (F1-F7) from a projection unit situated between the rolling stands (F1-F7), and capturing an image of the light and dark pattern with an image capture unit situated between the rolling stands (F1-F7); and a step of measuring the flatness of the sheet by analyzing the acquired pattern image, wherein if L 0 represents a distance in a horizontal direction between an intersection of respective optical axes of the projection unit and the image capture unit and, of the adjacent rolling stands (F1-F7), the rolling stand (F1-F7) that is equipped at the opposite side of the intersection from a looper that controls tension of the sheet between the rolling stands (F1-F7), α represents an angle formed between a vertical direction and a straight line formed by projecting the optical axis of the image capture unit onto a plane that contains a direction parallel to a rolling direction and the vertical direction, a value of α being positive if the optical axis of the image capture unit is inclined from the vertical direction toward the looper side, β represents an angle formed between the vertical direction and a straight line formed by projecting the optical axis of the projection unit onto the plane, a value of β being positive if the optical axis of the projection unit is inclined from the vertical direction toward the rolling stand side, hc represents a distance in the vertical direction between a pass line, which is a straight line passing through crest points of respective lower side work rolls of pairs of upper and lower work rolls that are respectively equipped at the adjacent rolling stands (F1-F7), and an image principal point of a lens of the image capture unit, and hp represents a distance in the vertical direction between the pass line and an image principal point of a lens of the projection unit, then L 0, α, β, hc and hp satisfy the following mathematical expression. 0.75 L 0 ≤ tan α + tan β 1 / h C − 1 / h P ≤ 1.5 L 0 a step of acquiring a pattern image by projecting a light and dark pattern composed of light portions and dark portions onto a surface of the sheet that is traveling between adjacent rolling stands (F1-F7) from a projection unit situated between the rolling stands (F1-F7), and capturing an image of the light and dark pattern with an image capture unit situated between the rolling stands (F1-F7); and projecting a light and dark pattern composed of light portions and dark portions onto a surface of the sheet that is traveling between adjacent rolling stands (F1-F7) from a projection unit situated between the rolling stands (F1-F7), and capturing an image of the light and dark pattern with an image capture unit situated between the rolling stands (F1-F7); and a step of measuring the flatness of the sheet by analyzing the acquired pattern image, wherein if L 0 represents a distance in a horizontal direction between an intersection of respective optical axes of the projection unit and the image capture unit and, of the adjacent rolling stands (F1-F7), the rolling stand (F1-F7) that is equipped at the opposite side of the intersection from a looper that controls tension of the sheet between the rolling stands (F1-F7), α represents an angle formed between a vertical direction and a straight line formed by projecting the optical axis of the image capture unit onto a plane that contains a direction parallel to a rolling direction and the vertical direction, a value of α being positive if the optical axis of the image capture unit is inclined from the vertical direction toward the looper side, β represents an angle formed between the vertical direction and a straight line formed by projecting the optical axis of the projection unit onto the plane, a value of β being positive if the optical axis of the projection unit is inclined from the vertical direction toward the rolling stand side, hc represents a distance in the vertical direction between a pass line, which is a straight line passing through crest points of respective lower side work rolls of pairs of upper and lower work rolls that are respectively equipped at the adjacent rolling stands (F1-F7), and an image principal point of a lens of the image capture unit, and hp represents a distance in the vertical direction between the pass line and an image principal point of a lens of the projection unit, then L 0, α, β, hc and hp satisfy the following mathematical expression. 0.75 L 0 ≤ tan α + tan β 1 / h C − 1 / h P ≤ 1.5 L 0