A system for automatically determining principal tones of an accompaniment line supporting a melodic line comprises a plurality of principal tone inference engines which infer principal tones in accordance with different rules of inference from one another and a priority control which controls priorities among the plurality of inference engines to determine final principal tones in the accompaniment line. Each principal tone inference engine may include a condition checker which checks to see whether muscial conditions are met and a principal tone generator which generates principal tones. The priority control monitors the information supplied from the condition checkers to select final principal tones supplied from the principal tone generators. Some of the final principal tones may be dependent from the preceding principal tone while some others may be independent. There may also be provided a storage of the structure of music which is reflected in a series of the final principal tones. In an application, a melodic line monitor is provided so that an accompaniment line interacting with the melody will be generated.