PURPOSE: To improve cutting performance by effecting cooling and dust collecting at a time through an open air circulation method, by a method wherein a suction rotary blade is provided around a rotary driving shaft, an opening part is formed on a suction part of the rotary blade in a cover member and the dust collecting hole is opened at a discharge part of chips.

CONSTITUTION: A suction rotary blade 5 is provided around a rotary driving shaft 3, an opening part 6 is formed on a suction part of the rotary blade 5 in a cover member 4 and a dust collecting hole 7 communicating with a dust collecting device is provided into a discharge part of dust. The dust generated by cutting-off of concrete C is discharged immediately through the dust collecting hole 7. Simultaneously with cooling of heat to be generated with chips 11... by compressed air injected through a communication hole 14, the same is cooled by the open air also through opening parts 6, 6. A blade 10 also is cooled directly similarly.