A road traffic congestion degree estimation system includes smart plate readers that detect vehicles driven on a general road externally leading to a resort. The numbers of local vehicles and strange vehicles that currently exist in the resort are calculated, based on the number of vehicles approaching the resort, the number of vehicles receding from the resort, and information of smart plates of the detected vehicles. Furthermore, a prospective degree of traffic congestion on an expressway, which introduces the strange vehicles receding from the resort via the general road into areas where the strange vehicles are based, is estimated based on the above calculated numbers of local vehicles and strange vehicles. In this estimation, the number of strange vehicles contributes more greatly to an increase in the degree of traffic congestion than the number of local vehicles does.