An automatic music composing apparatus comprises: a memory that stores a plurality of chord-progression data in a specific key; an input device that inputs motif melody data; a selector that selects at least one chord-progression data from the memory; a detector that detects a key of the input motif melody data; a transposer that transposes the selected chord-progression data into the detected key in accordance with relation between the detected key and the specific key; and a melody data generator that generates a melody in the detected key in accordance with the input motif melody data and the transposed chord-progression data.
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