A workpiece diameter measurement method includes: detecting positions of a probe while relatively rotating an uncalibrated standard and a detector around a rotation center in a state where the probe is in contact with a circumferential face of the standard from one side in a displacement direction of the probe, detecting the positions of the probe while relatively rotating the standard and the detector around the rotation center in a state where the probe is in contact with the circumferential face from another side in the displacement direction, calculating the position of the rotation center based on the detected positions, relatively rotating a workpiece and the detector around the rotation center in a state where the probe is in contact with the workpiece from the other side, and calculating a diameter of a circumferential face of the workpiece.