There are provided a surface shape measuring method and a surface shape measuring device which can measure the diameter of a workpiece to be measured with high precision and high reproducibility and have excellent versatility. These method include: acquiring first shape data indicating a surface shape of the workpiece with a detector being disposed on one side across a workpiece while rotating the workpiece relatively to the detector around a rotational center; acquiring second shape data indicating the surface shape of the workpiece with the detector being disposed on the other side across the workpiece while rotating the workpiece relatively to the detector around the rotational center; and calculating a shape parameter defining the surface shape of the workpiece by collating the first shape data and second shape data. In calculating the shape parameter, a deviation of the detector from the reference line is calculated based on the collation result.
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