A monitoring system includes an axle weight measurer and a state estimator. The axle weight measurer detects a surface displacement of a road from a first captured image obtained by imaging the road when a vehicle passes at a predetermined spot of a structure having the road that the vehicle passes, and calculates an axle weight of the vehicle from the surface displacement and a displacement coefficient of the road. The state estimator generates an axle weight distribution from the axle weight calculated by the axle weight measurer, and estimates a deterioration degree of the structure, using the axle weight distribution.