PURPOSE: To finish envelope opening by passing an envelope through a cutter only one time, open efficiently without trouble and damaging the content, and prevent a lowering of sealing performance before opening.

CONSTITUTION: A cutter comprises: a horizontal plate 11; a perpendicular plate 13 attached to the upper surface of the horizontal plate 11; and a vertical blade 14 formed in the front edge of the perpendicular plate 13 along the vertical direction. The vertical blade 14 is formed at an acute angle or obtuse angle relative to the horizontal plate 11. The cutter further comprises a horizontal blade 12 which is formed in the front edge of the horizontal plate 11 along the diagonal direction relative to the backward direction from the vertical blade 14 of the perpendicular plate 13.