Abstract: A system for measuring the vibration frequency of a vibrating object useful for the adjustment of the vibration frequency of a piano string or like vibrating objects. In this system, the vibration of the vibrating object is converted to an electric signal, based on which signal is sought the difference between the vibration frequencies of any two adjacent harmonics of the vibrating object, i.e. the fundamental vibration frequency of the vibrating object is sought. The electric signal is led to a counter via a voltage-controlled variable band-pass filter. The middle frequency of the pass-band of this filter is controlled by a sweeper and varies continuously. For each agreement of this middle frequency of the pass-band with any one of the harmonics, the output of the filter presents a peak. The frequency corresponding to each peak is detected by a counter. The fundamental frequency is calculated from the difference between the frequencies corresponding to any adjacent peaks of the outputs of the filter.