Having an imaginary companion (IC) is an example of children's pretend play. However, most research regarding children's ICs is from Western cultures. In this study, the prevalence of ICs was assessed among Japanese children (2- to 9-year-old children, N = 800). The developmental (age), biological (sex), and environmental (birth order) effects on Japanese children's ICs were also assessed. Moreover, whether IC status can be an indicator of fantasy orientation in Japanese children was examined. The results revealed that the prevalence of the invisible friend was relatively rare, but the personified object was prevalent in Japanese children. Age and sex, but not birth order, significantly affected the prevalence of ICs in Japan. Moreover, IC status significantly indicated children's fantasy orientation. The results suggest that the characteristics of Japanese children's ICs are partly different from those in Western children. Social-cultural contexts can affect this difference.