A corrugated board sheet defect detecting device, a corrugated board sheet defect removing device and a corrugated board sheet manufacturing device are provided with: a radiating device (45) which radiates parallel light toward a corrugating medium (B) at a radiation angle ([theta]1) that is inclined through a predetermined prescribed angle relative to a perpendicular line (L1) perpendicular to asingle-faced cardboard sheet (D); an image capturing device (46) which captures an image of a portion of the corrugating medium (B) irradiated by the parallel light; a shadow image processing device (51) which defines light portions (W) and dark portions (G) in the direction in which the single-faced cardboard sheet (D) is being transported, on the basis of the image captured by the image capturing device (46); and an assessing device (52) which assesses the quality by comparing a length (W1) of the light portions (W) and a length (G1) of the dark portions (G) as defined by the shadow image processing device (51) with predetermined assessment values.