A box-making machine and a defective product removal device thereof include a quality determination unit (101) for determining quality of a corrugated cardboard sheet (S) conveyed on a production line of a corrugated cardboard box (B) through a predetermined inspection, a defective product removal section (51) for removing, from the production line, a corrugated cardboard sheet (S) having been creased on a surface thereof and slotted, a discrimination unit (102) for operating the defective product removal section (51) on a corrugated cardboard sheet (S) determined to be defective by the quality determination unit (101). Therefore, a defective sheet material can be appropriately removed and working efficiency can be improved.