OBJECTIVES: The importance of supporting advance care planning (ACP) by healthcare professionals is recognized worldwide, and assessing the outcomes, such as people's understanding and readiness for ACP, using an appropriate instrument is essential. We, therefore, developed a Japanese version of the Advance Care Planning Engagement Survey (ACP Engagement Survey; 15 items, 9 items, and 4 items), an international scale for assessing the progress of the ACP, and examined its validity and reliability. METHODS: The ACP Engagement Survey was translated into Japanese, back-translated, and culturally adapted, and the final version was reviewed by the author of the original version. Data on basic demographic information and ACP-related experiences were simultaneously collected as external criteria in an online survey of older adults with chronic diseases. The Cronbach's alpha was calculated to assess its internal consistency, and a retest was performed three days later to calculate the intra-class correlation coefficients (ICCs). RESULTS: A total of 200 respondents (mean age 70; 9.5% female) were included in the analysis. None of the items showed a ceiling effect, but several items did exhibit a floor effect. The factor structure was the same 2-factor structure as the original version, and both factors exhibited a high cumulative contribution rate. The Cronbach's alphas were 0.94 (15-item version), 0.91 (9-item version), and 0.95 (4-item version), and ICCs were of 0.88 (15-item version), 0.9 (9-item version), and 0.84 (4-item version). SIGNIFICANCE OF RESULTS: The Japanese version of the ACP Engagement Survey was confirmed to have very good reliability regarding both internal consistency and test-retest reliability. Together with the result of the item analysis, we can conclude that the Japanese version of the ACP Engagement Survey is sufficiently reliable to be utilized in interventional studies, and it has acceptable content validity, construct validity, and criterion-related validity.