A cut groove having a predetermined depth is formed with a cutting blade along a projected dicing line set on a workpiece. A first preliminary groove is cut in a measurement member and a relational expression with respect to a distance from the preliminary groove bottom and the width of the preliminary groove is determined. An amount of incision into the workpiece is set to be equal to or smaller than a predetermined value from a reference position and the thickness of the workpiece. A second preliminary groove is cut along the projected dicing line, and an image of the second preliminary cut groove is captured. The width of the second preliminary cut groove is measured, and the depth of the second preliminary cut groove is calculated from the width of the second preliminary groove and the relational expression.