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Some of unexpected fires may self-extinguish due to insufficient heat release or air supply and some may be discovered and controlled by occupants. Accordingly an automatic fire detection system plays an important role in cases where effective human detection measures couldn't be expected. In Japan, automatic fire detection systems have been obligatorily installed in buildings for specific uses having a floor space above a certain value as prescribed by the Fire Service Law. Automatic fire detection systems seem to be considerably effective but there are many cases where the bell of the control unit does not operate because the custodians of the buildings keep the bell switched off in order to avoid false alarms. The probabilities of giving false alarm per year per detector are 6.5% in smoke detectors and 0.8% in heat detectors. There seem to be two current trends in the solution of false alarm problems. One is "right detector in the right place" and the other is to increase the information content on which discrimination between genuine fires and fire simulating phenomena can be made. Several methods have already been put into practice to greatly reduce the probability of false alarm occurrence to l%/year detector, and much more sophisticated fire detection systems will be adopted in near future.