<p>Gait festination (GF) is one of the representative locomotion disturbances associated with Parkinsonism, which induces pulsion caused by increased stepping velocity. Fear of falling over and involuntary symptoms caused by GF can have a significant impact on quality of life. Considering previous interventional strategies for GF and their utility in daily life, physical intervention using a wearable system to assist postural disorder and the hastening phenomenon is necessary for patients with GF. The purpose of this study was to develop a wearable system that can assist the patient in extending the trunk and generating internal rhythm. Another objective was to confirm the effectiveness of the system for preventing GF by conducting a gait experiment in a Parkinsonism patient. To design an assistive method, we simulated lateral swing in the standing position based on features of Parkinsonism, and defined the mechanism and power configurations for the system. A power unit at the back of the waist transmits a cyclic force to the chest by a link in the frontal plane. A gait experiment was performed in a Parkinsonism patient with GF to confirm that reduction of the gait cycle is prevented by using the system. The result showed that the linear trendline slope of reduction of the gait cycle decreased. In addition, the maximum stooping angle decreased by half, and the cyclic assistance of the system and gait cycle were harmonized. These results suggest that the system developed is effective in preventing GF by supporting posture and generating internal rhythm.</p>