PURPOSE: To recognize the specific part of an object to be recognized from roundness by forming light cutting plane shape on the object to be recognized by irradiating an almost cylindrical shape object to be recognized with light in a direction intersecting orthogonally to the longitudinal direction of the object, and finding the roundness by finding a circular arc approximated to the light cutting plane shape.

CONSTITUTION: When a seedling 3 as the object to be recognized is illuminated with slit light 5 from a slit light source 4 arranged on a placing board 2, the light 5 is formed in the light cutting plane shape along the shape of the seedling 3. When the cutting plane shape is image-picked up with an image pickup device 6, an image signal from the device 6 is sent to an image processing device 10, and is A/D-converted 11, then, it is stored in an image memory 12. Next, a pre-processing part 14 performs the smoothing and thinning processings of image data, etc., with the command of a main control part 13. And the circular arc approximated to each of the light cutting plane shape S1, S2,... Sn of obtained cutting plane shape is found with a circular arc matching part 15, and the roundness of the circular arc is found with a roundness calculating part 16, then, the shape of the specific part of the seedling 3 can be obtained from the roundness.