DEVICE AND METHOD FOR DETECTING DEFECT IN CARDBOARD SHEET, AND CARDBOARD BOX MANUFACTURING MACHINE

According to a defect detecting device and a defect detecting method for a corrugated cardboard sheet and box making machinery. The corrugated cardboard sheet includes one surface formed with a recessed portion of a first score in a thickness direction, and another surface formed with a recessed portion of a second score in the thickness direction. The recessed portion of the second score has a depth smaller than a depth of the recessed portion of the first score. The defect detecting device for the corrugated cardboard sheet includes a measurement device disposed opposing the one surface or the other surface and being configured to measure a distance from an arrangement position to a front end of the corrugated cardboard sheet and a distance from the arrangement position to the recessed portion of the first score or the recessed portion of the second score to acquire a measured distance, a storage device configured to store a threshold value that is preset, and a determination device configured to perform comparison of the measured distance with the threshold value in a determination region that is predetermined and preset from the front end toward a back end of the corrugated cardboard sheet, specify a magnitude relationship between the measured distance and the threshold value by the comparison, and determine a front/rear surface defect of the corrugated cardboard sheet or a front/back side defect of the corrugated cardboard sheet by using the magnitude relationship.