The authors propose a new evaluation method for 'Generalized Confidential Modulation (GCM)' for quantum communication. Confidential modulation realizes a secret communication by using secret information for modulation and noise in a channel. Y-00 is one of the famous methods of GCM for quantum communication. The existing evaluation methods for GCM are based on stream ciphers. They can estimate its analytical security and the evaluation depends on the security status of Pseudo Random Number Generator (PRNG) which controls the modulation. On the other hand, their method is based on mode of operation for block ciphers and clears the weaknesses from structural viewpoint.