PURPOSE: To satisfactorily perform a linear or curved cutting treatment, and beautifully finish the cutting surface by using, a cutting blade having a knife-like edge part having a substantially triangular cross sectional form.

CONSTITUTION: A current is applied to an ultrasonic generator 8 to supply a high frequency energy to an oscillator 3, and the energy is imparted as an ultrasonic vibration to a cutting blade 6 through a vibrating horn 4 and a connecting element 5. The cutting edge 611 of the cutting blade 6 is pushed onto a frozen fish and reciprocated to cut the fish. The cutting blade 6, which has a straight edge form having a substantially wedge-like cross section, is formed out of an edge part 61 and a back part 62. The edge part 61 has the acute angle cutting edge 611, which is formed in such a manner that the edge thickness is gradually increased toward the back part 62. A flank part is formed on the side wall of the back part 62 of the cutting blade 6. Therefore, the cutting surface of the cut frozen fish never make contact with the side wall. Consequently, the reciprocation of the cutting blade 6 is smoothly executed, and the cutting treatment can be performed with a high sharpness.