The flange cover covering the blade is divided into two. The second cover is turnably connected to the first cover through a shaft parallel with the spindle of the blade, and the second cover is opened and closed in connection with vertical movement of the spindle. When the spindle is moved upward for replacing the blade, the second cover is automatically turned to the opening position, and L-shaped coolant jetting nozzles are moved away from positions below the blade to positions beside the blade. Thus, the blade can be easily replaced, and an automatic blade replacing system can be applied.