Objective：The movement of an elderly person involving stepping over a bathtub makes them prone to falls and requires caution. Gait analysis suggested that the risk of falling increases with time spent in supporting both legs. A similar relationship was expected for side-straddling motion. This study aims to analyze differences in the side-straddling movements between THA patients who did not fully recover their functional balance, mobility, or walking ability from the viewpoint of the ratio of time spent supporting both legs.Methods：Eleven patients with hip osteoarthritis underwent initial unilateral THA via a posterior approach. Using a portable three-dimensional motion analyzer, we calculated the percentages of time spent supporting both legs while entering the bathtub, and exiting the bathtub with a handrail and examined the factors associated with percent of the time spent supporting both legs.Results：The percentage of time spent supporting both legs when exiting the bathtub was significantly lesser when exiting using the nonoperative than the operated side. A negative correlation was found between the percentages of time spent supporting both legs. while exiting the bathtub, and the percentage of handrail load values.Conclusion：The risk of falling may be reduced by using the nonoperative leg as the leading leg when exiting a bathtub. Use of handrails contributes to a reduction in fall risk.