Abstract: A device for measuring the frequency or period of an input signal independent of different frequency noise signals which may be superimposed on the input signal includes a filter circuit having a variable cut-off frequency. The output level of the filter circuit is detected by a level detector, the output of which electrically controls a variable element such as a variable resistor, a variable capacitance diode or the like which is included in the filter circuit. This controls the cut-off frequency of the filter circuit, so as to maintain its output level at a given level. In this manner, an input signal frequency is converted into a corresponding level, utilizing a declining portion adjacent to the cut-off frequency of the frequency response of the filter circuit. The converted level is utilized to vary the cut-off frequency of the filter circuit to maintain its output level constant, thereby automatically changing the cut-off frequency in accordance with the input signal frequency.