To simply cut the side plate of a liquid paper container without being injured by providing a guide plate and a cutting plate having a cutting member on the lower face in a truncated chevron shape at the tip of a handhold section.

This cutter 9 is constituted of a cutting plate 3 integrally injection-molded with polypropylene, a guide plate 4, and a handhold section 1, and the cutting plate 3 and the guide plate 4 are formed at an angle of 90°, for example. A cutting member 2 provided on the cutting plate 3 is integrated by insert injection molding, and it is formed perpendicularly to the cutting plate 3 so that the blade is directed to the moving direction of the cutter 9 to cut the side plate of a liquid paper container. When the liquid paper container is to be broken up with the cutter 9, the guide plate 4 is put on the bottom plate of the liquid paper container while the handhold section 1 is held, the cutting plate 3 is put on the side plate of the liquid paper container, the cutter 9 is pulled while being pressed from above, then the side plate of the liquid paper container is cut.