A portable tuning device samples discrete values on a fundamental frequency component of an audio signal, which is equivalent to sound waves produced in a musical instrument, and extracts plural series of fundamental frequency components to be converted to plural bit strings of 1s and 0s; since a time delay equal to the inverse of target frequency is introduced between the first bit of one bit string and the first bit of the next bit string, a series of gradation data has a bit string identical with the bit strings at the consistency with the target frequency, and the series of gradation data has bit strings different from the bit strings at the inconsistency regardless of a cycle time so that user recognizes the tuning state from the bit strings.