BackgroundThis study aims to develop a low-stakes assessment tool to establish a classroom English language benchmark that Japanese teachers of English can use for their own professional development purposes. To start with, we describe the differences between CLA (Classroom Language Assessment) in Hong Kong and the IDS (Integrative Diagnostic Scale) in Japan with regard to agenda, characteristics, and implementation. Then, we report findings made from both of these assessment types and from a group discussion we had in order to clarify the rationale behind the CLA experts’ assessment.MethodsThe participants of the present study consisted of two groups: (1) four project members of the IDS in Japan and (2) two CLA raters in Hong Kong. Following an initial assessment of videotaped performances by the IDS developers (based on the IDS benchmark) and assessment of the same videotaped performance by the CLA experts (based on the CLA), a follow-up focus-group discussion between the IDS developers and the CLA experts was held.ResultsThe results of the study offer some evidence to support the thesis that a classroom English benchmark helps capture aspects of classroom English language proficiency that are indispensable for students’ language acquisition. Accordingly, on the basis of the findings, suggestions for further revisions of the IDS were made.ConclusionThe study showed that, with some revisions, the IDS can be an apt professional development tool through which Japanese teachers of English can improve their classroom English proficiency. It also suggested that more research is needed to contextualize the IDS as such a professional tool more fully.