PURPOSE: To automatically detect and set an optimum aperture by comparing a delay calculated from a phase detecting output from a phase detector and an aperture value derived from an aperture table, with a decided value determined in advance.

CONSTITUTION: A signal to be measured is converted to an intermediate frequency signal by an oscillation signal from a local oscillator 2 by a mixer 1 of a frequency converting circuit 12, and inputted to a phase detector 3. An arithmetic means 5 calculates a delay from an output of the phase detector 3 at the time when the oscillation frequency of the local oscillator 2 has two different frequencies and an aperture value which is read out of an aperture table 7. A deciding means 6 compares the delay calculated by the means 5 and a decided value determined in advance, and compares its magnitude. A measured frequency setting means 9 inputs a desired measured frequency f0, and a local oscillator frequency control means 8 outputs its control signal to the local oscillator 2 so as to output two different oscillation frequencies from the local oscillator 2 based on the desired measured frequency f0 which is inputted from the means 9 and the aperture value which is read out of the aperture table 7.