PURPOSE: To adjust easily an input signal to a prescribed frequency with high accuracy, by using also a microcomputer without using a frequency counter.

CONSTITUTION: A beat waveform B is obtained by inputting an input signal A to an amplifier circuit 2 with a limit of gain and mixing the signal at a reference signal generating circuit 1. A signal waveform C is obtained by passing this waveform B through a reference frequency eliminating circuit 5. The input signal A is changed into a signal D having f0 of frequency and a maximum output level by passing the signal waveform C through an LPF6, where f0 is the frequency to be adjusted for the input signal A and the frequency of the circuit 1. The signal D is converted into a digital signal at an AD converter 7 and given to a CPU9 through an input/output circuit 8. The input signal passes through a phase detecting circuit 4 to recognize whether the frequency of the input signal A is larger or smaller than the f0 and it is discriminated whether the output is H or L level and the result is given to the circuit 8. Thus, the frequency adjustment with high accuracy is attained with the comparison of the basic frequency.