To easily change the setting of the unit feed amount of a screw type feed mechanism to a feed amount corresponding to a cutting condition or the like, make manufacture advantageous in the point of assembly work efficiency in an execution work site and in the point of a construction cost, and efficiently and easily operate initial adjusting work at a mounting time in an existing pipe.

A cutter comprises a cutting unit 3 having a cutting tool 2, a guide support part 4 supporting the cutting unit 3 to freely rotate around a pipe axial core X, and a drive part 5 driving and rotating the cutting unit 3, the cutting unit 3 is provided with a screw type feed mechanism 6 moving the cutting tool 2 in the pipe diametric direction and a passive rotary unit 9 giving a rotation corresponding to the unit feed amount to the screw type feed mechanism 6 by engagement with an engaging part 7 provided in a fixed part, a speed reduction mechanism 10 is interposed in a rotation transmitting system to a screw shaft 8 of the screw type feed mechanism 6 from the passive rotary unit 9, and the other end part of a rotary shaft of the speed reduction mechanism 10 is formed of a reverse rotation operating part moving the cutting tool 2 returned to an opposite side to the pipe axial core X of an existing pipe 1.