PURPOSE: To prevent water from going into the corrugated card-board made of thermoplastic resin, by forcing a heated blade having a specified shape to the corrugated card-board so that the resin is softened and melted to form a film to the cut end of the corrugated card-board thereby sealing quickly the cut end.

CONSTITUTION: For example, the blade 2 with a slant 4 inclined relatively to the sheet surface of corrugated card-board 1 is heated and driven down toward the corrugated card-board 1 of a thermoplastic resin placed on a platen 3 so that the tip of the blade 2 is forced on the sheet surface, and as the blade 2 is forced into the partly softened and melted corrugated card-board 1, a softened, melted film 6 of the thermoplastic resin is formed along each side of the slant 4 of the blade 2. When the blade 2 is lowered until the tip 7 reaches the undersurface of the corrugated card-board 1, the corrugated card-board 1 is cut into corrugate card-board pieces 8, 9, and each of the cut ends is sealed by a softened and melted film 6 that extends from the surface sheet 1a and the undersurface sheet 1b. Finally the softened and melted film is cooled and solidified to complete the sealing.