OBJECTIVES: Undocumented immigrants (UIs) have been reported to suffer from the unequal distribution of COVID-19 vaccination, but this inequality has never been quantified, and the associated factors have not been measured. STUDY DESIGN AND METHODS: We interviewed 190 municipal offices throughout Japan about the access to COVID-19 vaccination for UIs and control group foreigners. Using logistic regression, we investigated the association between assured access and municipal characteristics. RESULTS: Out of the respondent municipalities, 57.5% answered that UIs can apply for a COVID-19 vaccination voucher. Additionally, 31.5% said they had received an inquiry about vaccines from UI individuals. Furthermore, only 23.2% of the municipalities responded that they had issued vouchers for UIs at least once. The control groups were reported to have been given more access to vouchers. Logistic regression showed that the foreign resident ratio, tertiary industry, and university graduation ratio were positively associated with vaccination access. CONCLUSIONS: This study revealed for the first time that UIs are disproportionately marginalized compared with other visitors, implying that "illegality" plays an important role in the context of vaccination eligibility. The street-level vaccination desks of local governments may refuse to supply vaccines. Vaccine equity will be more readily achievable when vaccination access to all populations including UIs is ensured. Such access will also improve overall public health by increasing the vaccination rate.