PURPOSE: To easily replace a rotary blade without detaching a front cover by constituting a blade cover of the front cover and a rear cover and positioning the front cover to an open state and a closed state.

CONSTITUTION: When a rotary blade 3 is replaced, a setscrew 7 is loosened to spring up a front cover 2a held to a closed state to bring the same to an open state. Whereupon, a cutting soln. supply nozzle 4 retreats upwardly to get out of the way. When the adjusting screw 13 of a blade monitor part 2c is let retreat to be moved upwardly, a blade detection part 14 retreats from the rotary blade 3 to get out of the way. By this constitution, the rotary blade 3 can be easily replaced by detaching an attaching nut 5. After the rotary blade 3 is replaced, the front cover 2a is rotated downward and the setscrew 7 is screwed in a screw hole 8 to again return the front cover 2a to the closed state.