An abnormality detection system includes an optical fiber, a backscattered light detector, and a data processor. The backscattered light detector is connected to one end side and the other end side of the optical fiber, and configured to acquire a first intensity distribution of backscattered light by making light incident on the optical fiber from the one end side and to acquire a second intensity distribution of backscattered light by making light incident on the optical fiber from the other end side. The data processor calculates transmission loss at each position in the longitudinal direction of the optical fiber by using the first and second intensity distributions and a normalization function, and determines whether or not there is an abnormality based on the result of the calculation.