To surely determine abnormality in the number of bundled corrugated fiberboard bundled by a corrugated fiberboard bundler.

A pressing cylinder for pressing a corrugated fiberboard bundle from the upper surface when the bundler bundles has a length measuring means. A method of determining abnormality in the number of bundled sheets in the corrugated fiberboard bundler is for determining whether the height of the bundle is within the predetermined allowable range based on the measured length value by the length measuring means. The method includes a step of calculating the average measured length value as for predetermined n bundles regarded to be not abnormal (where n is a positive integer) measured just before the bundles as an object to be determined and a step of determining whether the measured length value of the bundles as the object to be determined is within the allowable range with the average value as reference.