The previous literature suggests that familial relationships play a fundamental role in developing social preferences, beliefs, and opinions. In particular, some studies have found that the sex of the first child has a causal effect on opinions about gender issues, which is referred to as the first-daughter effect. This paper provides evidence on the first-daughter effect in Japan with the most serious gender disparity among developed countries (World Economic Forum (2021)). Our estimation results show the average first-daughter effect to increases public support for the opinion “The best way for women to be independent is to have a job.” Additionally, significant heterogeneity is revealed by employing the causal inference with machine learning.