A tuning device includes an analog-to-digital converter for converting sound waves propagated from a piano to an audio signal in synchronism with a sampling signal and a data processor for visualizing a difference between a target pitch of a tone and an actual pitch of the tone; since the sampling signal is less accurate in frequency, the data processor previously determines the frequency difference between a design frequency and an actual frequency of the sampling signal by using a calendar clock signal as a measure; the data processor eliminates noise due to the sampling signal from visual images of the pitch difference, and makes the visual images more accurate.