In this paper, we proposed a novel conditional access system to reduce the overhead transmission rate for messages which verify the entitlement of subscribers in bandwidth-limited system. We adapted a key update system with the tree structure which had been used for IP multicast to a group hierarchy for the subscriber groups or channel groups. We also analyzed the overhead transmission rate for periodic key update and aperiodic one for change in a subscriber. The numerical evaluations show that the proposed system can significantly reduce the overhead transmission rate for dynamic subscriber change up to several hundred thousands times for various system configuration.