This study was designed to examine the development of writing proficiency in Japanese and English of JHL (Japanese as a Heritage Language) speakers, and investigate the relationship between acquiring writing skills in the two languages from the viewpoint of bilingualism. Twenty-one adults participated. Data were collected via questionnaires to obtain the participants' language background and writing samples in Japanese and English using one essay topic. The essays were analyzed quantitatively. The results confirmed Cummins' interdependence hypothesis for writing in both languages if the writers were in the additive state. Although a variety of significant correlation coefficients were observed between the categories of WE (Writing Expertise; content and organization) in Japanese and those of LP (Language Proficiency; vocabulary, grammar, and mechanics) in English, no notable relationships were seen between the same categories. The interdependence hypothesis in non-cognate languages such as Japanese and English does not work solely on the basis of surface individual linguistic knowledge.