A defect inspection system for a wood plank, the defect inspection system comprising: a light source for visible light that emits a white-light for reflected light onto one side of a wood plank; a light source for invisible light that emits near-infrared-light for transmitted light onto another side of the wood plank that is opposite the one side; a second light source for visible light that emits visible light for transmitted light onto the another side of the wood plank, the visible light for transmitted light having a color that is easily discriminated from a color of reflected white-light that is from the light source for visible light and is reflected by the wood plank; a capture device that generates a reflected light image of the visible light reflected by the one side of the wood plank, a transmitted light image of the near-infrared-light transmitted through the wood plank, and a second-visible-light image of the visible light transmitted through wood plank; and an image processing device that detects a plurality of kinds of defects of the wood plank by analyzing a captured image generated by the capture device, wherein the image processing device determines the presence and type of any of the plurality of kinds of defects in the wood plank on a basis of the characteristics of a set that includes: shading and shapes that reflects the characteristics of transmitted near-infrared-light for each type of defect of the wood plank in the transmitted light image, and colors, shapes and patterns that reflects the characteristics of reflected light for each type of defect of the wood plank in the reflected light image, and colors and shapes that reflects the characteristics of transmitted light for each type of defect of the wood plank in the second-visible-light image. a light source for visible light that emits a white-light for reflected light onto one side of a wood plank; a light source for invisible light that emits near-infrared-light for transmitted light onto another side of the wood plank that is opposite the one side; a second light source for visible light that emits visible light for transmitted light onto the another side of the wood plank, the visible light for transmitted light having a color that is easily discriminated from a color of reflected white-light that is from the light source for visible light and is reflected by the wood plank; a capture device that generates a reflected light image of the visible light reflected by the one side of the wood plank, a transmitted light image of the near-infrared-light transmitted through the wood plank, and a second-visible-light image of the visible light transmitted through wood plank; and an image processing device that detects a plurality of kinds of defects of the wood plank by analyzing a captured image generated by the capture device, wherein the image processing device determines the presence and type of any of the plurality of kinds of defects in the wood plank on a basis of the characteristics of a set that includes: shading and shapes that reflects the characteristics of transmitted near-infrared-light for each type of defect of the wood plank in the transmitted light image, and colors, shapes and patterns that reflects the characteristics of reflected light for each type of defect of the wood plank in the reflected light image, and colors and shapes that reflects the characteristics of transmitted light for each type of defect of the wood plank in the second-visible-light image. shading and shapes that reflects the characteristics of transmitted near-infrared-light for each type of defect of the wood plank in the transmitted light image, and colors, shapes and patterns that reflects the characteristics of reflected light for each type of defect of the wood plank in the reflected light image, and colors and shapes that reflects the characteristics of transmitted light for each type of defect of the wood plank in the second-visible-light image.