To always and constantly maintain the frequency of an intermediate frequency signal to be outputted including a moment when the frequency of a high-frequency pulse signal to be inputted changes by inserting a delay circuit to a high-frequency pulse signal side and constantly performing tuning without any delay between the two input signals in a mixer.

While a high-frequency pulse signal is branched into two portions by a distributor 1, frequency is instantly measured and frequency data are outputted. A local oscillator 3 outputs a local oscillation signal. The other branched high-frequency pulse signal is inputted to a delay circuit 4, is delayed, and then is inputted to a mixer 5. The delay circuit 4 has a delay time that is equal to time being obtained by adding the measurement time of an instantaneous frequency measuring equipment 2 and time until a local signal is generated after inputting frequency data by the local oscillator 3. Mixing is made with the local signal of the local oscillator 3. A specific intermediate signal is selected and outputted from the output signal of the mixer 5 by a filter 6. The intermediate frequency signal retains a constant frequency that does not follow the frequency change of an input high-frequency pulse signal.