A portable tuning device includes a microphone, a data processing system and a touch-panel liquid crystal display device, and assists a user in a tuning work on an upright piano; when the user depresses a key, sound waves, which expresses a tone, are propagated to the microphone, and are converted to an audio signal, the waveform of which is converted to audio data codes; the data processing system firstly calculates an introductory autocorrelation on the audio data codes for a small number of delayed waveform so as to determine a register to which the tone belongs, and, thereafter, calculates a principal autocorrelation on the audio data codes for delayed waveforms within the register so as to estimate the pitch of the tone; since the compass is narrowed to the register through the introductory autocorrelation, the amount of calculation is relatively small in the principal autocorrelation.