We disclose a scale thickness measuring method of measuring a thickness of scale attached to an outer peripheral surface of a heat transfer pipe having a cylindrical shape centered on an axis by using an eddy-current probe, which is provided with an excitation coil generating an eddy current, and a pair of detection coils detecting axial and circumferential components of a magnetic field formed by the eddy current, the scale thickness measuring method including: obtaining a reference point in a region to which the scale is not attached by calculating a difference between the axial and circumferential components of the magnetic field detected by the pair of detection coils; moving the eddy-current probe inside the heat transfer pipe in the direction of the axis while inserting the eddy-current probe into the heat transfer pipe; calculating the difference between the axial and circumferential components of the magnetic field; obtaining a drift amount by comparing the difference with the reference point; and calculating the thickness of the scale from a calibration curve obtained in advance on the basis of the drift amount.