Abstract: Based on a desired center frequency and a frequency resolution for a signal to be measured which are entered via an operator section, a micro-CPU calculates the frequency dividing number, a preset value and a center value for the signal to be measured. The frequency dividing number is set in a frequency divider, which frequency divides the signal to be measured and provides the frequency-divided output to a continuous period-to-voltage converter. The continuous period-to-voltage converter counts clock pulses occurring in each frequency-divided period, measures the difference between fractions of time from both the beginning and the end of the frequency-divided period to the generation of the next clock pulses, respectively, and outputs, as a voltage corresponding to the frequency-divided period, the sum of a voltage corresponding to a predetermined number of consecutive low-order bits of a binary value, obtained by adding the preset value to the pulse count value, and the fractional time difference.