Actual grinding depth measurement method

In a machining method of supporting a workpiece (W) having a cylindrical machined portion such that the workpiece (W) is rotatable and feeding a grinding wheel (7) in a radial direction, a start diameter (D0) that is a diameter including a measurement start point on a surface of the machined portion is measured, and, after the measurement start point passes through a machining application portion, an end diameter (D1) that is a diameter including a measurement end point is measured. An actual grinding depth (U) at the time when the measurement start point is machined is computed by the equation, U = |D0 - D1|, a runout of the machined portion is computed from a relative difference in the actual grinding depth (U) between positions of the machined portion in a rotational direction, and infeed control of the grinding wheel (7) is executed such that the runout is removed.