Starting with Hashimoto's work in 1928 and continuing until recent times, the prevailing belief was that the reflex of the phoneme /*p/ of Proto-Japanese was a bilabial fricative [ɸ] as early as the Nara period. This perspective had widespread acceptance. In this paper, I intend to outline the key studies on this topic, particularly those that have critiqued Hashimoto's hypothesis. Special emphasis will be given to Wenck’s theory from 1959, which, despite receiving limited recognition within Japanese scholarship, has significantly contributed to the fields of Japanese philology and phonological research. Moreover, to support the theory suggesting [p] as the most archaic phonetic value, I will provide a detailed account of the reflexes of /*p/ and the underlying diachronic changes in the contemporary language varieties found in the Ryūkyū Islands. Ultimately, this paper will conclude by identifying the phonetic values of the discussed phoneme in Old and Late Middle Japanese, both at the word initial and word medial positions, based on the most viable and phonologically plausible theories available.