A corrugated board sheet defect detecting device detects a defect in a single-faced cardboard sheet guided by a guide member with corrugated core paper facing outwards. The device includes a radiating device, an image capturing device, an image processing device, and an assessing device. The radiating device is configured to radiate light toward the core paper at a radiation angle relative to the single-faced cardboard sheet. The image capturing device is configured to capture an image of a portion of the core paper irradiated with the light. The image processing device is configured to a light portion and a dark portion in a direction in which the single-faced cardboard sheet is transported based on the captured image. The assessing device is configured to assess a quality by comparing a length of the light portion and a length of the dark portion.