PURPOSE: To obtain an image excellent in an S/N based on the combination with a linear light source and to improve a recognition ratio by using marked characters consisting of segments parallel with the visual field of an one- dimensional camera.

CONSTITUTION: A marked character 1 is marked by a laser marking device 2 and constituted of only radial straight lines obtained from the rotational center 4 of a doughnut plate 3. The marked character 1 is always parallel with the visual field 6 of the one-dimensional camera 5 which is obtained by rotating the doughnut plate 3 around the rotation center 4 and includes the extended line of the rotational center 4.