PURPOSE: To extract the extending direction of a stem at the high position part of the stem by irradiating the stem at a prescribed angle with the continuous light of a photoirradiating means in a direction almost intersecting orthogonally to the extending direction of the stem of a seedling such as a potato, etc., and obtaining the light cutting plane shape of the stem.

CONSTITUTION: When the seedling 3 of the potato is arranged on a placing board 2 and the seedling 3 is irradiated with slit light 5 from a slit light source 4, the light 5 is formed in the light cutting plan shape along the shape of the seedling 3. The light cutting plane shape is image-picked up with an image pickup device 6, and an image signal from the device 6 is A/D-converted 11 at an image processing device 10, and is stored in an image memory 12. Next, a pre-processing part 14 receiving a command from a main control part 13 performs the thinning processings of the light cutting plane shape L1 produced by irradiating the seedling 3 with the light 5 and parts L2 and L3 produced by irradiating the plane of the placing board 2, and calculates a distance L from the seedling 3 to the device 6 by using the light cutting plane shape pre- processed at a distance calculating part 15, then, the extending direction of the stem can be extracted at a stem direction extracting part 17.