The Japanese writing system employs two distinct categories of characters: Kana and Kanji. The difference between Kana and Kanji writing corresponds roughly to that between phonological and lexical systems in Western languages. When typing, most Japanese use alphabetical characters based purely on phonological rules. In particular, the Romaji system is used, in which a syllable consists of a single vowel, a consonant–vowel compound (e.g., ka, ki), or a sequence of consonant–y(semivowel)–vowel (e.g., kya, kyu). We describe a right-handed Japanese patient who developed pure agraphia that affected Romaji writing but preserved Kana and Kanji writing and who had a lesion in the left pars opercularis of the inferior frontal gyrus that extended to the anterior precentral gyrus. The patient demonstrated literal paragraphia in spelling Romaji across modalities. Our findings suggested that the patient’s agraphia in Romaji after a confined left frontal infarction was manifested by a selective impairment in syllable-to-grapheme conversion.