A method for measuring flatness of a sheet includes acquiring a pattern image by projecting a light and dark pattern composed of light portions and dark portions onto a surface of the sheet, which is traveling between adjacent rolling stands, from a projection unit situated between the rolling stands, and capturing an image of the light and dark pattern with an image capture unit situated between the rolling stands, and measuring the flatness by analyzing the acquired pattern image. Arrangement parameters L0, α, β, hc and hp satisfy the following mathematical expression (1). 0.75 ⁢ L 0 ≤ tan ⁢ ⁢ α + tan ⁢ ⁢ β 1 / h C - 1 / h P ≤ 1.5 ⁢ L 0 ( 1 )