Importance Vaccination is a critical measure to contain the COVID-19 pandemic. To persuade the public of Japan, a vaccine-hesitant nation, is essential. Objective We survey determinants that affect Japanese acceptance of vaccines against COVID-19. Design We conducted a randomized conjoint analysis survey on preference over choice of a vaccine. The survey assigned respondents 5 choice tasks. In each task, respondents evaluated 2 hypothetical COVID-19 vaccines and were asked whether they would choose vaccines A or B. The vaccine attributes included efficacy, major and minor adverse side effects, country of vaccine development and clinical trial, and vaccine type. Setting We conducted the survey on the Internet. Participants A nonprobability sample of 15,000 Japanese adults. Treatment A choice task asked participant to select a vaccine from 2 hypothetical ones as optional or to make which one mandate, with a probability of 0.5 for each. Main Outcomes and Measures We expected that country of development and clinical trial, vaccine type, and whether the vaccination is optional or compulsory would affect the preferences. Results Domestic and US development compared to Chinese development raised the probability of choice by 131% (average marginal potential outcome increased from 0.29 to 0.67) and 96% (to 0.57), respectively. A domestic clinical trial increased it by 33% (from 0.43 to 0.57). A rise in efficacy from 50% to 90% increased it 43% (from 0.41 to 0.59). A decrease in the risk of severe side effects from 1 per 10 thousands to 1 per 1 million increased it by 38% (from 0.42 to 0.58). Vaccine type was irrelevant. Making vaccination compulsory raised the acceptance probability of China- and Russia-developed vaccines by 6% and 4%, respectively, and raised that of taking a high-risk vaccine by 5% and a modestly effective (70%) vaccine by 4%. General vaccination hesitancy, political positions, and demographic characteristics were irrelevant. Conclusions and Relevance A domestically development or clinical trial would substantially increase willingness to take the vaccine. Making vaccination compulsory would modestly reduce penalty on a vaccine with side effects, geopolitical, and efficacy concerns. The tendency is common across background characteristics, including attitudes toward vaccination in general.