In developed countries, progressive rapid aging is increasing the need for social care. This study aimed to determine Japanese utility weights for the Adult Social Care Outcomes Toolkit (ASCOT) four-level self-completion questionnaire (SCT4).We recruited 1050 Japanese respondents from the general population, stratified by sex and age, from five major cities. In the best-worst scaling (BWS) phase, respondents ranked various social care-related quality of life (SCRQoL) states as "best," "worst," "second-best," or "second-worst," as per the ASCOT. Then, respondents were asked to evaluate eight different SCRQOL states by composite time-trade off (cTTO). A mixed logit model was used to analyze BWS data. The association between cTTO and latent BWS scores was used to estimate a scoring formula that would convert BWS scores to SC-QALY (social care quality-adjusted life year) scores.Japanese BWS weightings for ASCOT-SCT4 were successfully estimated and found generally consistent with the UK utility weights. However, coefficients on level 3 of "Control over daily life" and "Occupation" domains differed markedly between Japan and the UK. The worst Japanese SCRQoL state was lower than that for the UK, as Japanese cTTO results showed more negative valuations. In general, Japanese SC-QALY score (for more than 90% of health states) was lower than that for the UK.We successfully obtained Japanese utility weights for ASCOT SCT4. This will contribute to the measurement and understanding of social care outcomes.