PURPOSE: To securely fix a flat blade even if there is same machining error in the fitting part of the blade, in a ultrasonic cutter device for setting the flat blade to the cutter device and fixing it with screws.

CONSTITUTION: The fitting plane 2 of a flat blade 4 is formed in the horn 1 of a cutter device and a screw 5 having a recessed form at the front end is arranged opposite to the fitting plane 2. The flat blade 4 is placed on the fitting plane 2 and fastened with the screw 5. When setting the flat blade 4, the screw 5 is fastened to fix the flat blade 4 and further, the recessed front is deformed to fix the flat blade securely by fastening the screw 5. In this way, the supersonic vibration is securely transmitted and the cutting efficiency is increased.