Method for measuring a resonant frequency of a tool set into ultrasonic vibration during machining of a workpiece, comprising the steps of a) radiating a working signal with a working frequency through a generator into a tool holder with the tool for generating the ultrasonic vibration of the tool; b) after starting to machine the workpiece, radiating a test signal (St) with a test frequency varying by the operating frequency through the generator into the tool holder; c) generating a sensor signal from the ultrasonic vibration of the tool by a sensor device arranged in the tool holder; d) reading out the sensor signal by a readout device; e) breaking down the sensor signal into a frequency spectrum, which has a main frequency and a secondary frequency, by an analysing device; f) determining the main frequency of the frequency spectrum from the operating frequency and determining the resonant frequency from the secondary frequency of the frequency spectrum; and g) increasing a vibration amplitude of the ultrasonic vibration of the tool; characterised in that the machining is a machining of the workpiece, the test signal (St) has a lower power (Pt) than a power of the working signal and increasing the vibration amplitude of the ultrasonic vibration of the tool is performed by one or more of the following steps: g2) increasing the power of the working signal by a second control means based on a height of a peak in the frequency spectrum at the working frequency; and g3) increasing an oscillation amplitude of the working signal by a second control means based on a difference between the working frequency and the measured resonant frequency. a) radiating a working signal with a working frequency through a generator into a tool holder with the tool for generating the ultrasonic vibration of the tool; b) after starting to machine the workpiece, radiating a test signal (St) with a test frequency varying by the operating frequency through the generator into the tool holder; c) generating a sensor signal from the ultrasonic vibration of the tool by a sensor device arranged in the tool holder; d) reading out the sensor signal by a readout device; e) breaking down the sensor signal into a frequency spectrum, which has a main frequency and a secondary frequency, by an analysing device; f) determining the main frequency of the frequency spectrum from the operating frequency and determining the resonant frequency from the secondary frequency of the frequency spectrum; and g) increasing a vibration amplitude of the ultrasonic vibration of the tool; the machining is a machining of the workpiece, the test signal (St) has a lower power (Pt) than a power of the working signal and increasing the vibration amplitude of the ultrasonic vibration of the tool is performed by one or more of the following steps: g2) increasing the power of the working signal by a second control means based on a height of a peak in the frequency spectrum at the working frequency; and g3) increasing an oscillation amplitude of the working signal by a second control means based on a difference between the working frequency and the measured resonant frequency. g2) increasing the power of the working signal by a second control means based on a height of a peak in the frequency spectrum at the working frequency; and g3) increasing an oscillation amplitude of the working signal by a second control means based on a difference between the working frequency and the measured resonant frequency.