We have developed a novel visual sensing system that monitors respiration for 24 hours non-restrictively. The principle of the system is inter-image subtraction. Compared with conventional methods, the system has advantages which make it suitable for a long-term monitoring: 1) patients experience no physiological burden during sleep, 2) no manual operation is necessary since it has self-optimizing functions for image processing and 3) it judges patient's sleep condition adequately using statistical analysis. The system thus can be applied to predict abnormality of health conditions, and thus for example, alert care providers so that action can be taken with a view to preventing the sudden death of elderly people or, in case of children, to prevent occurrence of sudden infant death syndrome. It can be also applied to the detect clinically dangerous sleep apnea syndrome. Experimental results in a nursing home for the aged have shown the efficiency of the system.