PURPOSE: To detect a green knot accurately by irradiating the surface of a lumber veneer with beltlike light, and detecting the constriction of a beltlike pattern of transmitted light appearing on the reverse surface through the lumber and outputting a knot detection signal responding to the signal of the part.

CONSTITUTION: A cylindrical lens 25 which converges light from a light source 24 in a beltlike shape and the lumber veneer 26 are shown in a figure and the veneer 26 is sent as shown by an arrow. The knot 27 of the lumber veneer 26, transmitted light pattern 28 appearing on the reverse surface of the veneer 26, an image forming lens 29, and the 1st and the 2nd one-directional line sensors 30 and 31 arranged on the image formation surface of the image forming lens 29 are shown. Analog signals from those line sensors 30 and 31 are coded into binary signals by binary-coding blocks 32 and 33 and inputted to an OR gate 34, which outputs the knot detection signal.