PURPOSE: To improve the measuring accuracy of frequency modulation characteristic by outputting voltage corresponding to the frequency of a signal to be measured in a digital signal, and displaying it after converting it into an analog signal by signal-processing it by a delay circuit and a subtracter.

CONSTITUTION: When the signal 1 to be measured whose frequency changes linearly with time is inputted, an FVC 8 generates the voltage corresponding to the frequency of the signal 1 to be measured, and transmits it to an ADC 10. One side of the output signals 10a of the ADC 10 is transmitted to a digital subtracter 12, and the other side is transmitted to a digital delay circuit 11. The digital subtracter 12 detects frequency difference between the output signal 10a of the ADC 10 and the output signal 11a of the digital delay circuit 11, and transmits it to the DAC 13. The DAC 13 converts it into the analog signal, and transmits it to an oscilloscope 9. Thus, the instrument of the high measuring accuracy of the frequency modulation characteristic can be realized.