1. A music-piece processing apparatus comprising: a storage section that stores respective music piece data sets of a plurality of music pieces, each of the music piece data sets comprising respective tone data of a plurality of fragments of the music piece and respective character values of the fragments, the character value of each of the fragments being indicative of a musical character of the fragment; a designation section that designates, from among the plurality of music pieces stored in said storage section, one music piece as a main music piece and one or more music pieces as sub music pieces; a comparison section that compares the character value of each of the fragments of the main music piece designated by said designation section and the character value of each individual one of the fragments of the one or more sub music pieces designated by said designation section, wherein said comparison section calculates a similarity index value indicative of a degree of similarity, to the character value of each of the fragments of the main music piece, of the character value of each individual one of the fragments of the one or more sub music pieces; a processing section that, on the basis of results of the comparison by said comparison section, processes the tone data of each of the fragments of the main music piece on the basis of the tone data of any one of the fragments, similar in character value to the fragment of the main music piece, of the designated one or more sub music pieces, wherein said processing section determines, on the basis of the similarity index value calculated by said comparison section, similarity between the character value of each of the fragments of the main music piece and the character value of each individual one of the fragments of the one or more sub music pieces, and said processing section processes the tone data of a given one of the fragments of the main music piece on the basis of the tone data of any one of the fragments of the sub music pieces which has been determined to be similar to the given fragment; and a coefficient setting section that sets a coefficient for each of the one or more sub music pieces in response to operation by a user, wherein said comparison section includes an adjustment section that adjusts the similarity index values, calculated for the fragments of each of the sub music pieces, in accordance with the coefficient set by said coefficient setting section for the sub music piece, and said processing section determines, on the basis of the similarity index values adjusted by said adjustment section, similarity between the character value of each of the fragments of the main music piece and the character value of each individual one of the fragments of the one or more sub music pieces.