Data recently collected from some Japanese dialects such as Ibuki Island in the Inland Sea (Setonaikai) and neighboring areas in Kagawa prefecture, or Tsubata and Shiramine in Ishikawa prefecture, reveal a specific tonal pattern which bears a mid-tone mora (or a sequence of mid-tone morae) after 2 initial high tone morae in a single tonal phrase, as shown by #V V V …. This paper presents a hypothesis that this particular tonal pattern is derived, diachronically, from the sequence #HLH …, by rightward spread of the first high tone, thereby causing the downstepping of the second high tone, yielding the pattern #HHM….<br>Based on the assumption that phrases in the above geographical regions that now display #HHM… used to be, at a certain stage of their historical development, the sequence #HLH…, this paper first suggests a proto-accentual system of the mainland Japanese dialects, and subsequently illustrates the way in which some of the present dialects have developed their tonal systems from the common source. Contrary to the general assumption that the proto-accentual system of mainland Japanese dialects is that of the late Heian period annotated in the Ruijumyogisho, this paper attempts to show that the natural phonetic account of the tonal patterns of these dialects requires a common stage prior to that described in the Ruijumyogisho.<br>The paper further reconstructs a proto-accentual system of Japanese, based on the data collected in the present Ryukyuan dialects as well as the proto-accentual system of mainland Japanese established in the previous section of this paper. It is then suggested that, at least at one point, proto-Japanese had a balanced tonal-melody system consisting of extensive use of combinations of High and Low tones.