A method for measuring flatness of a sheet material in which a light and dark pattern composed by a light portion and a dark portion is projected onto a surface of the sheet material running in a longitudinal direction, a pattern image is acquired by photographing the light and dark pattern by an image pickup device having an image pickup visual field larger than a width of the sheet material, and the flatness of the sheet material is measured by analyzing the acquired pattern image, comprising: a first step of using a staggered pattern as the light and dark pattern projected onto the surface of the sheet material, the staggered pattern being composed of light portions arranged in a staggered form at a predetermined preset pitch in a longitudinal direction and a transverse direction, and projecting the staggered pattern onto the surface of the sheet material so that the longitudinal direction of the staggered pattern corresponds to the longitudinal direction of the sheet material and the transverse direction of the staggered pattern corresponds to the width direction of the sheet material; a second step of arranging the image pickup device at a position at which the specularly reflected light on the surface of the sheet material of the staggered pattern can be received, and acquiring the pattern image by photographing the staggered pattern by the image pickup device; a third step of setting a shape measurement line extending along the longitudinal direction of the staggered pattern at a predetermined position in the acquired pattern image; a fourth step of averaging picture element concentrations on a straight line which passes through a picture element on the shape measurement line and extends in the transverse direction of the staggered pattern, and has a length two times or more the transverse preset pitch between the light portions to calculate an average picture element concentration; a fifth step of calculating distribution of the average picture element concentrations along the shape measurement line; and a sixth step of calculating a surface shape of the sheet material along the shape measurement line based on the calculated average picture element concentration distribution, and calculating the flatness of the sheet material based on the calculated surface shape. a first step of using a staggered pattern as the light and dark pattern projected onto the surface of the sheet material, the staggered pattern being composed of light portions arranged in a staggered form at a predetermined preset pitch in a longitudinal direction and a transverse direction, and projecting the staggered pattern onto the surface of the sheet material so that the longitudinal direction of the staggered pattern corresponds to the longitudinal direction of the sheet material and the transverse direction of the staggered pattern corresponds to the width direction of the sheet material; a second step of arranging the image pickup device at a position at which the specularly reflected light on the surface of the sheet material of the staggered pattern can be received, and acquiring the pattern image by photographing the staggered pattern by the image pickup device; a third step of setting a shape measurement line extending along the longitudinal direction of the staggered pattern at a predetermined position in the acquired pattern image; a fourth step of averaging picture element concentrations on a straight line which passes through a picture element on the shape measurement line and extends in the transverse direction of the staggered pattern, and has a length two times or more the transverse preset pitch between the light portions to calculate an average picture element concentration; a fifth step of calculating distribution of the average picture element concentrations along the shape measurement line; and a sixth step of calculating a surface shape of the sheet material along the shape measurement line based on the calculated average picture element concentration distribution, and calculating the flatness of the sheet material based on the calculated surface shape.