To provide a cutoff device minimizing resistance to a blade of a circular saw, having the circular saw available for a long time, and preventing damage of the blade of the circular saw.

The circular saw 13 draws a circular arc moving locus and abuts on a square pipe W from obliquely above the square pipe to start cutting of the square pipe W. Since the circular saw 13 abuts from a square part Wa, the number of blades of the circular saw 13 simultaneously slid on the square pipe W is extremely few. Resistance to the blade of the circular saw M is then minimized, and also generation of frictional heat is suppressed. The blade is hardly worn in a short period of time, therefore, and deterioration of the blade by the frictional heat is also suppressed.