A fire alarm system, determining existence or nonexistence of a fire by using an ultrasound wave, comprises a sound wave generator and a sound wave detector to detect sound waves propagated through two propagation paths having different lengths each other. The system comprises a calculation means for calculating a pressure ratio between a first sound pressure, which is a sound pressure of a sound wave propagated through a first propagation path, and a second sound pressure, which is a sound pressure of a sound wave propagated through a second propagation path, and a smoke density estimator. The smoke density estimator calculates a change ratio between the pressure ratio calculated by the calculation means and a predetermined standard pressure ratio, and determines a smoke density from the change ratio based on a predetermined relational expression describing the relation between the change ratio and the smoke density, and determines existence of a fire when the smoke density exceeds a predetermined threshold.