To examine reliability, validity, factor structure, and measurement invariance (i.e., configural, metric, and scalar invariance) of the Japanese Conners’ Adult attention deficit hyperactivity disorder (ADHD) Rating Scales (CAARS), Japanese nonclinical adults ( N = 786) completed the CAARS Self-Report (CAARS-S). Each participant was also rated by one observer using the CAARS Observer Form (CAARS-O). For the test of measurement invariance, excerpts from the original (North American) CAARS norming data ( N = 500) were used. Dimensional structure was examined by exploratory and confirmatory factor analyses. Test–retest reliability, internal consistency, and concurrent validity were satisfactory. Based on the DSM-IV model and Japanese four-factor model, configural and metric invariance were established for the CAARS-S/O across Japanese and North American populations. Scalar invariance was established for the CAARS-O based on the Japanese model. The use of the Japanese CAARS for diagnostic purposes in Japan was supported; however, it should be used with caution for cross-cultural comparison research.