To provide a spindle structure for an ultrasonic processing machine capable of accurately processing by preventing thermal expansion of a supporting horn, and the supporting horn used for the same spindle structure.

The spindle structure for an ultrasonic processing machine comprises an ultrasonic vibrator 4 arranged in a hollow sleeve body 3 formed on a spindle 2, and the supporting horn 5 coaxially connected with the ultrasonic vibrator 4 and restrictedly fixed on an inner peripheral wall of the sleeve body 3 and transmitting ultrasonic vibration excited by the ultrasonic vibrator 4 to a cutting tool 6. The supporting horn 5 has a main body 5a coaxially connected with the ultrasonic vibrator 4, a base section 5b-1 formed at an outer peripheral surface of the main body 5a in a flange shape, a cylindrical buffer sleeve 5c-1 with elasticity formed by extending the outer peripheral section of the base section 5b-1 in the axial direction of the main body 5a, and a peripherally contacting flange 5d formed on an end of the buffer sleeve 5c-1 and fixed in a contact state with the inner peripheral wall of the sleeve body 3 by pulling the flange to the outside of the sleeve body 3 through the sleeve body 3.