Objectives: Recently, the Tokyo Metropolitan Assembly passed an ordinance prohibiting smoking in private homes and cars if children are present. However, no previous study has investigated existing, voluntary home and car smoke-free rules in Japan. Therefore, we examined prevalence and determinants of comprehensive home and car smoke-free rules. Design: A cross-sectional study. Setting: Internet survey data with adjustments using inverse probability weighting for ‘being a respondent in an internet survey’. Participants: 5600 respondents aged 15-69 years in 2015 were analyzed to estimate weighted percentages and prevalence ratios (PRs) with 95% confidence intervals (CIs) of having comprehensive home and car smoke-free rules. Main outcome measures: Respondents who answered ‘smoking is never allowed’ in their home or car were defined as having home and car smoke-free rules. Results: Overall, 47.0% (95% CI = 45.8–48.3) of respondents implemented comprehensive home and car smoke-free rules. People who agreed with ‘smoking relieves stress’ were less likely to have comprehensive smoke-free rules (PR=0.76 [0.71–0.82]), especially among ever-users of electronic nicotine delivery systems (PR=0.49 [0.30–0.81]). Higher education was significantly associated with higher PR for comprehensive smoke-free rules (PR=1.30 [1.19–1.41]). Living with children was significantly associated with higher PR for smoke-free rules among current smokers than not living with children. Among never and former smokers, electronic device ever-users were less likely to have smoke-free rules than non-users (PR=2.91 [1.99–4.27]). Conclusions: In Japan, about 50% of respondents had voluntary smoke-free rules in the home and car. Information on current voluntary smoke-free rules will be useful as baseline information on home and car smoke-free status before enforcement of the 2018 Tokyo home and car smoke-free legislation.