We study the levels and characteristics of people's willingness-to-pays (WTPs) for the Covid-19 vaccine under various situations, by conducting a survey experiment on a nationwide sample in Japan and using multiple hypothetical questions with Multiple Price List formats. Specifically, we conduct the following three studies. In Study 1, we examine how the levels of the WTPs change according to vaccine effectiveness, infection stage, and vaccination progress. We then conduct a follow-up survey, capture their actual vaccination behavior, and clarify the correlation between their WTPs and vaccination behavior. In Study 2, we examine how the WTPs are influenced by nudge-based messages emphasizing selfish or altruistic benefits of the vaccination. In Study 3, we compare the levels and characteristics of the WTPs between young people (25-39 years old) and elderly people (60-74 years old). We further explore how the WTPs’ differences depend on the distributions and parameters of the variables of socio-economic attributes and behavioral economic and psychological characteristics, including time preferences, risk preferences, social preferences, and subjective risks.