To eliminate defects of a saw and rotary cutter used so far for cutting off a synthetic resin pipe such as a polyethylene pipe.

A pair of frame members 1 provided with connecting piece parts on both sides of an arcuate rotary blade support part 2 for supporting projectingly a rotary blade 5 on the recessed part side is constituted. The respective frame members have a joining screw rod 6 connected to one side connecting piece part 3 while having the other side connecting piece part 4 formed with a through hole 7 for the joining screw rod. Recessed parts of the rotary support parts are opposed to each other, and a pair of frame members are disposed so that the joining screw rods of the respective frame members are passed through the through holes of the other side frame member and constituted to be fastened by fastening nuts 8. Two rotary blades 5 are supported at an interval in the rotary support parts of at least one side frame member while a rotary cutter constitutes a handle 9 for cutting-off operation in a proper position.