A cutting unit includes a spindle, a spindle housing, a support flange on the front of the spindle, a cutting blade detachably supported to the support flange, and a fixing flange for fixing the cutting blade to the support flange. The support flange includes a boss portion adapted to be inserted through a central opening of the cutting blade, a flange portion for supporting one side surface of the cutting blade, and a cylindrical portion for engaging the front of the spindle. The flange portion has a suction hole opening toward the fixing flange, and the cylindrical portion has a communication hole communicating with the suction hole. The communication hole is connected to a vacuum source through a rotary joint fixed to the spindle housing so that communication between the communication hole and the vacuum source is selectively made by an on-off valve.