Defect detecting device and defect detecting method for corrugated cardboard sheet, and box making machinery

A defect detecting device for a corrugated cardboard sheet, the corrugated cardboard sheet including 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 having a depth smaller than a depth of the recessed portion of the first score, the defect detecting device comprising:

• 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;
• and
• a determination device configured to perform comparison of the measured distance with a threshold value that is preset 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, wherein
• the threshold value is preset based on a distance from the arrangement position to a design value depth of the recessed portion of the first score according to specifications of the corrugated cardboard sheet, or a distance to a design value depth of the recessed portion of the second score according to the specifications of the corrugated cardboard sheet.