A morphed musical piece generation system that enables even a user with little knowledge of music to easily generate a morphed musical piece between two different musical pieces is provided. A first intermediate time-span tree data generation section 6 selectively removes difference information between common time-span tree data and first time-span tree data from the first time-span tree data.Also, a second intermediate time-span tree data generation section 7 performs the same operation to obtain second intermediate time-span tree data. A data combining section combines the first intermediate time-span tree data and the second intermediate time-span tree data to generate combined time-span tree data. A musical piece data generation section generates a morphed musical piece on the basis of the combined time-span tree data.