1. A method of quantifying vulnerability of a system, comprising: converting each of the vulnerability identification results of the system into a vulnerability score so that the corresponding vulnerability identification results of the system can be applied to calculation of scores; calculating a target organization security level score corresponding to the system based on a technology-field security level score and a management-field security level score among the vulnerability scores; converting status of a local network of the system being separated from an external network into a network separation score; calculating an interim score based on the target organization security level score and the network separation score; and quantifying the vulnerability of the system by finally calculating a composite score of the system using the interim score and a simulated intrusion success level.