PURPOSE: To obtain a detector with which the signal can be extracted with high responsiveness regardless of the form or travelling direction of a travelling object, by increasing the brightness of the light volume distribution of illumination within the detecting range of the travelling object toward the edge from the center part of the distribution.

CONSTITUTION: The light volume distribution of illumination is increased toward its edge from the center part within a detecting range of a travelling object. For instance, a travelling object (subject) 1 is fed to a pair of feed rollers 3 and 3' existing right before a photographing part via another pair of feed rollers 2 and 2'. Meanwhile a subject detecting light source 5 illuminates the entire subject path 6 via a slit plate 9. The reflected light from the subject 1 is condensed to a photodetector 8 via a fan-shaped optical guide plate 7 to be detected in case the subject exists. Then the photographing is carried out by the signal of detection. A slit 9a of the plate 9 is made slimmer at its center part than the circumference part to increase relatively the transmitted light volume at the circumference part in comparison with the center part.