PURPOSE: To cause the output frequency of a signal generator and an AM receiving frequency to agree with each other automatically in a short time by sample-holding the output signal of a sample AM receiver by the A/D converter and the D/A converter of a signal detection part and by converting the output signal to a digital signal to control the frequency of the signal generator.

CONSTITUTION: Frequency transitions +Δf and -Δf are given to output frequency f of the fixed-amplitude rario frequency band alternately at a fixed period in frequency transition part 2, and the frequency is adjusted to a desired level by variable attenuation part 3, and signals f+Δf and f-Δf where the frequency is transited at a fixed period and applied to sample AM receiver 4. Then, the output signal of receiver 4 is subjected to sample holding by A/D converter 51 and D/A converter 52 of signal detection part 5 and is converted to a digital signal, and motor 6 is driven to control frequency f of generator 1, thereby making frequency f equal to the receiving frequency. As a result, the output frequency of generator 1 and the AM receiving frequency can be caused to agree with each other automatically in a short time at a characteristic measuring time of the AM receiver.