Abstract: By the side of a powdery or granular material stacked in a conical pile on a horizontal surface, a sensor that emits sensor light and, by receiving light reflected from a spot illuminated by the sensor light, detects the distance to the pile of the powdery or granular material is placed with the optical axis of the sensor light horizontal. While the sensor is moved in a horizontal direction perpendicular to the optical axis of the sensor light, the distance to each of a plurality of measurement points arranged horizontally at regular intervals along a line that runs on said pile of the powdery or granular material so as to face the movement path of the sensor. This step is performed every time the sensor is moved vertically a predetermined distance at a time. In each session of the step, a point is extracted that is closest to the sensor, and a function describing a straight line approximating the curve through all of the thus extracted points is determined by the least-squares method.