Unobservability in network communication is an important safeguard for hiding user's private activities. However, existing solutions including mix systems only offer anonymity, which is lower-level concept than unobservability. In this paper, we introduce a new mix model that effectively provides the unobservability by generating fake messages efficiently even in the environment where traffic volume may be low or the bounded transmission delay of messages is important. That is, users can hide their communication and determine the delay of message transmission. Also, it is possible to detect and isolate areas where blending attack occurs.