This paper proposes a method of evaluating the N-1 security quantitatively with increased PV penetration. The authors applied the concept of Robust Power System Security (RS) proposed by us to this method. By using Robust Dynamic Security region (RDS), which is based on this concept and the key region for judging the security, we can evaluate the power system security as follows. (1) RDS is identified as the benchmark of the N-1 security. (2) Yearly declining trend of the N-1 security is shown in Japan. (3) The necessary amount of additional power supply capacity can be calculated to maintain the future N-1 security same as the present level. The effectiveness of the proposed method is demonstrated through simulations using a three machines model system.