Automatic composition apparatus and method are provided which are capable of modifying a rhythm pattern to provide a wide variety of rhythm patterns only through simple setting, and a storage medium that accomplishes this function is provided. Characteristics of a rhythm pattern of a piece of music to be composed are established. Characteristic data that match the established characteristics are retrieved from a rhythm pattern database rhythm pattern database comprising a plurality of rhythm patterns and characteristic data indicating characteristics of each of the rhythm patterns. One of the rhythm patterns is selected, that corresponds to the retrieved characteristic data. Accordingly, different rhythm patterns having the same characteristics can be easily obtained. Further, composition conditions to be applied to a whole piece of music to be composed are established, and the established composition conditions are allocated to each of sectional elements that constitute the piece of music, for example, a passage, a phrase, or a measure, and the composition conditions for the whole piece of music, in other words, distributed among the respective passages, phrases or measures. Therefore, a rhythm pattern having different characteristics may be obtained by changing only the characteristics of the whole piece of music, thus assuring an improved efficiency with which the user operates the apparatus.