A cutting unit includes a spindle housing, a spindle, a mounter fixed to the front end portion of the spindle for supporting a cutting blade, and a fixing flange for fixing the cutting blade to the mounter. The mounter includes a boss portion for axially slidably supporting the fixing flange, the boss portion being fitted to an engaging hole formed in a central portion of the fixing flange, a flange portion projecting radially outward from the outer circumference of the boss portion, the flange portion having a supporting surface for supporting the cutting blade on the front side opposed to the fixing flange, and a cylindrical portion formed on the rear side of the flange portion. The flange portion has a suction hole opening. The cylindrical portion has a communication passage communicating with the suction hole. The communication passage is connected through a rotary joint to a suction unit.