PURPOSE: To segment images on the surface of an object at high speed by segmenting images by linearly arranging a lot of fine-area shutter elements which are opened when light more than the prescrived quantity of light is received.

CONSTITUTION: When there is any flow on the surface of an iron plate, light is irregularly reflected on the head part of the flaw in the moving direction of the iron plate, and one part of reflected light is made incident from the lower part of an image segmenting device 8 to a position detecting element 4a of the device 8. As the result, the electric resistance of a light receiving element 2a in the element 4a, to which the reflected light is made incident, is decreased, a voltage to be impressed to an optical modulating element 3 serially connected to this element 2a is increased, and this element 3 is turned to a light transmitting state. Therefore, the reflected light transmits through the element 3, is made incident to one position detecting element in a quantization device installed at the upper part of the device 8 and operates the quantization device. Thus, the images are segmented.