To solve the problem wherein existing instruments configured to slit containers such as milk cartons and then to cut the bottom cannot safely, reliably and easily slit containers such as cartons and PET bottles different in shape and size for cutting the bottom.

An instrument includes a backing part 14 and a slitting blade 8. The backing part 14 has an abutting surface 33 capable of abutting a surface of a container. The slitting blade 8 has a cutting edge 15 for driving the slitting blade 8 into the container surface. The cutting edge 15 is spaced from a plane defined by the abutting surface 33 of the backing part 14.