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 (S1) with a working frequency (f1) 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 (f1) through the generator into the tool holder ; c) generating a sensor signal (S2) from the ultrasonic vibration of the tool by a sensor device arranged in the tool holder ; d) reading out the sensor signal (S2) by a readout device ; e) breaking down the sensor signal (S2) 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 (f1) and determining the resonant frequency (f2) 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 (P1) 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 (P1) by a second control means based on a height of a peak in the frequency spectrum at the working frequency (f1); and g3) increasing an oscillation amplitude of the working signal (S1) by a second control means based on a difference between the working frequency (f1) and the measured resonant frequency (f2). a) radiating a working signal (S1) with a working frequency (f1) 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 (f1) through the generator into the tool holder ; c) generating a sensor signal (S2) from the ultrasonic vibration of the tool by a sensor device arranged in the tool holder ; d) reading out the sensor signal (S2) by a readout device ; e) breaking down the sensor signal (S2) 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 (f1) and determining the resonant frequency (f2) 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 (P1) 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 (P1) by a second control means based on a height of a peak in the frequency spectrum at the working frequency (f1); and g3) increasing an oscillation amplitude of the working signal (S1) by a second control means based on a difference between the working frequency (f1) and the measured resonant frequency (f2). g2) increasing the power of the working signal (P1) by a second control means based on a height of a peak in the frequency spectrum at the working frequency (f1); and g3) increasing an oscillation amplitude of the working signal (S1) by a second control means based on a difference between the working frequency (f1) and the measured resonant frequency (f2). <EOS>