This study is an attempt to scale the ability of Japanese 9th graders to use relative pronouns of English language, utilizing the difference in the degree of response construction required by three item formats. Subjects responses were scored into three categories according to their qualitative differences: correct relative pronoun, wrong relative pronoun, and other or no answer. Bock's nominal response model yielded a scale with a large amount of information, despite that the scale consisted of only 7 items. This scale could be characterized as a continuum from being unable to give a relative pronoun even when an item explicitly required one, to an ability to know the necessity of a relative pronoun in a composition, and the proper selection. The subjects ability estimates on this scale might be useful in planning further effective instructions.