To provide a method and an apparatus for rapidly exactly automatically cutting a box such as a corrugated board box housing a content or the like without deteriorating working environment and the quality of the content.

A thin-plate like round blade type rotatable blade 20 is pushed in an outer circumferential surface of a horizontally supported box 4 by predetermined cut depth H1 smaller than the diameter of the rotatable blade 20 and is circulated one time along the outer circumferential surface of the box 4 keeping the cut depth H1 approximately constant while horizontally rotating the blade so that the front margin in the advancing direction faces the outer circumferential surface of the box 4 whereby the upper portion of the box is cut.