The authors have defined Evaluation factor such as octave factor and consonant interval factor based on overtones to evaluate the quality of classical guitar tone. These Evaluation factor have made us possible to quantify the difference in sound between a high-end handcrafted guitar and an inexpensive guitar. It also became clear that the consonant interval factor, which evaluates the guitar sound by dividing it into consonant intervals and dissonant intervals elements is more rigorous than the octave factor, which focuses only on octave notes, in evaluating tone quality. This paper developed an evaluation system for classical guitar tone that uses the consonant interval factor. An evaluation device was fabricated and its operation was tested, enabling the calculation of evaluation factor in real time. A quantitative evaluation method for the tone of musical instruments has not yet been established. We believe that this tone evaluation system will enable us to construct a consistent evaluation method for tones that are often evaluated by the senses.