The invention relates to an existing pipe cut-off method and slitting method, by which an existing pipe made of cast iron and steel can be cut off. With a cut-off method according to the invention, first, a part of an existing pipe 1 is enclosed and sealed up by a seal-up housing 2 in an airtight state, a cutting tool 30 having a plurality of blades 30c is accommodated in the corresponding seal-up housing in a state the cutting tool is attached to the abovementioned seal-up housing 2. Next, the cutting tool 30 is fed toward the center of the abovementioned existing pipe 1 while slitting the existing pipe 1 by rotating the cutting tool 30 by power of a prime mover, and the abovementioned existing pipe 1 is cut off by the cutting tool 30 by turning the abovementioned seal-up housing 2 in the circumferential direction R of the existing pipe 1.