PURPOSE: To ensure easy judgement of directivity by a simple mechanism and to manufacture a device inexpensively, by drawing a polygon by straight lines, and scanning arbitrary points on the sides of the polygon by a plurality of sensors, without using high technology.

CONSTITUTION: A polygon is drawn on a paper surface S by drawing parts A and B. The paper surface S is moved so that scanning lines cross all the 8 drawing lines, and sensors and the drawing pattern are correctly positioned. Then, a slider 11 is moved so that 8 intersections are formed by the scanning lines and the drawing pattern. The presence or absence of the drawn lines or the difference in light and shade at each intersection is read by the sensor, and a pulse signal is generated. When the 8 drawn lines are correctly described, 8 pulse signals are generated. Even one drawn line is missing or faint, the sum of the pulse signals become fewer than 8. Therefore, the quality of the drawn lines can be judged by the pulse number.