This paper shows how the so-called lower case katakana writing is utilized for the transliteration of loanwords in Japanese, avoiding unpermitted phonological sequences. In order to adapt the foreign ti, di, tu, du, hu sequences, Japanese orthography employs lowercase writing (i. e., written in “small letters”) to preserve the phonological entity of the target words in the transliteration, e. g., disko “disco” → <deisko>, feis “face” → <ɸue:su>. That is, the diphthongal representations have to depend on special symbols, i. e., lowercase glides. Due to many complexities, Japanese writing system, especially the Romanization, is regarded as one of the most complicated systems which cannot be accounted for in a simple way. In order to provide a unified account on this issue, we employ the framework of Optimality Theory and show what kinds of constraints and their ranking relations are required the Japanese lowercase writing. We here claim that vowel correspondence is ranked higher than consonantal correspondence in Japanese lowercase writing. Moreover, the preservation of mora is another important factor in loan adaptation. Furthermore, the constraint ranking is different, depending on the period of adaptation; the new and old loanwords are realized differently from each other, although they have the same phonemic inputs.