Abstract: A tonality extractor analyzes a chord progression (CP) to determine an available pitch class set (PCS) at each chord time of CP using stored musical knowledge base. The available PCS is determined differently depending on musical situations. A resultant PCS succession is most accurate and assures a natural stream of musical tonality free of a spurious modulation. An automatic accompaniment apparatus utilizes the tonality extractor for desired accompaniment performance. A modulation compensator avoids generation of a pitch range shift in the accompaniment line at the time of modulation (change of key). A style-grouped pitch change table in combination with a style-grouped reference accompaniment pattern memory optimizes accompaniment pitch contents to respective musical styles while requiring only a small storage space.