PURPOSE: To measure tones stably with high accuracy and to perform the production of marker sounds and fault displaying of an electric power source surely by measuring the tones digitally.

CONSTITUTION: After the sound signal produced by a tuning string converted by a microphone is amplified by an amplifier 2, the pulse wave signals synchronized with basic wave components are outputted from a synchronous oscillator 3. The 1st counter 14 generates an output when it counts said pulse wave signals by as much as the predetermined number, and the 2nd counter 32 counts reference clock pulses for measurement. The counting of the 2nd counter 32 is continued over the period from the count starting of the 1st counter 14 until the generation of the output by a control part. The integrated value of the 2nd counter 32 is displayed in a display part 35, and when a sounding switch 34 is closed, the frequency divided output of the 2nd counter 32 is supplied to a sounding body driving part, by which the marker sound of a reference scale is obtained. When the source voltage drops below the reference value, a detection circuit operates and a display device 35 flashes to display this.