Prevention of falling during walking or transfer is important for maintaining quality of life in the elderly. Analysis of fall among the elderly is useful not only for preventing fall, but also to find out disease in its early stage. The aim of this study is to develop an ambulatory monitor that records falling time and frequency for long period. The monitor consists of a fall sensor, analog and digital circuits including a microprocessor. A photo-interrupter is used as the fall sensor. The photo-interrupter outputs a trigger signal when a subject fall and then the microprocessor records the falling time. To minimize the power consumption, the photointerrupter is driven with pulse current, and the microprocessor that records falling time is only operated when the subject falls. The monitor operates continuously for one week. The monitor was evaluated for a nor-mal subject during daily activity and it operated without any trouble in total 120 hours. The monitor was also evaluated for nine hemiplegia patients during rehabilitation training and it detected a falling.