A defect detecting device includes a detection unit and a movement mechanism. The detection unit includes a support, an arm supported by the support swingably around a swing shaft, and a sensor for detecting a displacement of the arm relative to the support. The arm includes contactors. The movement mechanism keeps the contactors away from a feed plane during a feed phase of a sheet panel, and moves the contactors to the feed plane and then moves the contactors from the feed plane during a pause phase of the sheet panel. The defect detecting device further includes a determination part configured to determine whether a defect in bag making is present based on data from the sensor.