An ultrasonic horn used in, for instance, a wire bonding apparatus and formed with mounting flanges, including a slit and a cross-sectional shape varying portion; the slit being on the central axis in the horn's longitudinal direction and extending fore and aft relative to the center of the mounting flanges, the length of the slit(s) being equal to or greater than the width direction of a flange region that is between the opposing flanges, and at least a part of the cross-sectional shape varying portion being on the outer surface of the horn at a slit region in which the slit is formed. The stress center point in the cross-section of the ultrasonic horn at the flange region is positioned more to the inside than a straight line joining the stress center points in the cross-sections of the ultrasonic horn at the front and rear end portions of the slit.