PURPOSE: To detect and display the deviation of the frequency of a musical sound signal from the closest scale frequency by varying the cutoff frequency of one filter and filtering the input musical sound signal.

CONSTITUTION: The musical sound signal which is inputted through a microphone 6 and an amplifier 11 is inputted to a filter part 13 through a switching circuit 12 and the filter part 13 varies the cutoff frequency sequentially and performs the filtering process. A control part 14 detects the cutoff frequency of the maximum-level output as the frequency of the musical sound signal and the cutoff frequency of the filter part 13 is fixed at this detected frequency. Then plural scale signals of scale frequency a scale frequency generation part 16 are inputted from a scale frequency generation part 16 to the filter part 13 and the control part 14 detects the scale frequency of the maximum-level output of the filter part 13 as the scale frequency closest to the musical sound frequency. The control part 14 calculates the deviation of the frequency of the musical sound signal from the scale frequency and displays it out on a display device 20.