Disclosed is a method of measuring the OF width of a cylindrical single crystal ingot using an optical non-contact type displacement measuring device. This method can avoid the labor and measurement errors as generally seen in the conventional manual method and provide an easy and precise measurement. Further, the method can be automated if necessary. This method comprises the steps of detecting the boundary points between the OF and the round surface of the single crystal ingot from the displacement to be obtained by scanning the sensor while detecting the distance between the sensor and the ingot surface including the OF, and calculating the OF width C from the scanning distance A of the sensor from the first boundary point detection position to the second boundary point detection position and the difference B between the first distance 1.sub.1 from the first boundary point to the sensor and the second distance 1.sub.2 from the sensor to the second boundary point.