To eliminate the possibilities that a reinforcing core material might come off or be shaky and thereby, increase the strength of the folded part of a plastic corrugated fiberboard beyond the normal level by inserting the reinforcing core material into the folded part of the plastic corrugated fiberboard and monolithically welding the core material to the liner of the plastic corrugated fiberboard as a single piece.

The plastic corrugated fiberboard 1 comprises connected hollow parts 1b partitioned with a center core between two synthetic resin liners 1a, and a reinforcing core material 2 of thermoplastic resin charged into the hollow parts 1b. In addition, a foldable groove 5 is formed in one of the liners 1a and the reinforcing core material 2 of the plastic corrugated fiberboard 1 and is folded, and at the same time, the cutout of the foldable groove 5 is closed together by thermal welding. This folding structure includes the reinforcing core material 2 in the foldable part, which is welded to the liner 1a of the plastic corrugated fiberboard 1 as a single body. Thus it is possible to prevent the reinforcing core material 2 from becoming dislocated from the liner 1a or becoming shaky.