This paper proposes a trajectory planning method using a Bezier curve for arriving aircraft in terminal airspace. This method enables aircraft to avoid severe weather areas and adjusts the time of arrival to maintain safety intervals between aircraft. In recent times, air traffic density has seen continued growth. However, when severe weather conditions occur in a terminal airspace with heavy traffic, aircraft are required to avoid these areas. This causes a delay in the expected time of arrival and risks shortening the safety interval to the following aircraft. In such a case, to maintain the safety interval, the following aircraft is required to adjust its arrival time by flying a longer path when it avoids severe weather areas. However, as air traffic density increases, it becomes more difficult for air traffic controllers to determine appropriate and feasible paths, and this increases their workload. Therefore, to reduce the workload of air traffic controllers and realize safer and more efficient air traffic management, a real-time severe-weather-avoidance trajectory planning method that enables the adjustment of arrival time is required. To solve this problem, this paper proposes a real-time trajectory planning method using a rational Bezier curve.