A weight measuring device includes a displacement amount detector, a storage unit, and a weight calculator. The displacement amount detector detects, by using a captured image obtained by capturing a road and a vehicle present on the road, a displacement amount, in the captured image, corresponding to displacement caused on the road by application of a weight of the vehicle. The storage unit stores first information indicating a relation between the weight and the displacement amount. The weight calculator calculates the weight based on the displacement amount and the first information.