1. A similarity assessment method of a performance sound, comprising: a probability density function generating process of dividing one performance sound into a sequence of frames each having a predetermined temporal length and also dividing another performance sound to be compared with said one performance sound into another sequence of frames each having the predetermined temporal length, and generating a probability density function of a fundamental frequency for each frame of the respective performance sounds; and a similarity assessment process of comparing the probability density function of a frame of said one performance sound with the probability density function of a frame of said another performance sound so as to assess a similarity between said one performance sound and said another performance sound, wherein the probability density function generating process uses a plurality of tone models which simulate various harmonic structures of sounds generated from a musical instrument and defines a weighted mixture of the tone models corresponding to various fundamental frequencies, and recurrently updates and optimizes respective weight values of the tone models so that a frequency distribution of the weighted mixture of the tone models represents frequency components of the performance sound, thereby outputting the optimized weight values as the probability density function of the fundamental frequency of the performance sound.