CPU 21 decides a current melody tone CM relating to a key depressed at a the leading position of a current beat and a previous melody tone PM relating to a key depressed at the leading position of a beat coming directly before the current beat, based on time information, in particular, beat information, controlling progress of automatic accompanying data in operation in a melody sequence progressing in response to manipulation of a keyboard 11. Further, CPU 21 performs a chord name deciding process to decide a current chord name, based on the decided current melody tone, previous melody tone, and a previous chord name PreCH, or a chord name at the previous beat. When deciding a melody tone, CPU 21 decides the current melody tone CM and the previous melody tone PM based on what number the current beat is in a measure.