According to Official Guinness Records,
The best selling single by a Japanese act is "Oyoge! Taiyaki-kun" ("Swim! Taiyaki") which was released by Masato Shimon on 25 December 1975. It spent 11 weeks at No.1 in Japan, selling over 4.5 million copies.
For a complete list of 1975 records, please visit 1975 Guinness Records in Japan.