Ungulates exhibit various and persistent behavioral responses to anthropogenic disturbance. However, information about natural behavior of sika deer with regard to their flight response is lacking. To clarify the effects of internal and external factors on the flight response of sika deer, it is crucial to initially evaluate this response in areas with low anthropogenic disturbance. Therefore, this study aimed to investigate the flight distance (FD) of sika deer in response to human presence on a forest road and clarify the influences of both internal and external factors on FD. Although there was no significant difference in the number of deer observed per 1 km between the fawn-rearing and rutting seasons, the numbers of males and females significantly varied seasonally, with more females observed in the fawn-rearing and more males in rutting season. FD was significantly influenced by herd composition, herd size, deer positioning, and terrain ruggedness index, indicating that females with fawn, larger herds, and deer in uphill or areas with high terrain ruggedness index tended to flee farther distances. Consequently, given the influences of internal and external factors on FD, as demonstrated in the present study, continuous monitoring of FD could provide valuable insights into wildlife management.