In this chapter, we present a formulation for protecting digital library as intellectual property, especially image digital library. The entire content of digital library assembled by database designers is to be differentiated from its individual contents. The digital library community demands an innovative approach for protecting digital library associated with content-based retrieval that dynamically generates indexes to its contents. The entire content with dynamically assigned indexes goes beyond the scope of the conventional copyright protection of the database with statically assigned indexes. The proposed formulation uses the patent of content-based retrieval process, and protects its object digital library in the specified domain without any excessively exclusive protection in general domains. That formulation determines whether the problem retrieval process identifies a classification of the entire content stored in its object digital library as a single and unique collection, or its equivalents within the scope of its specified domain. The similar collection realized in other digital libraries evidences unauthorized use of the problem retrieval process, or its equivalents, as far as it is patented. The patent of content-based retrieval process works as a catalyst of digital library protection, and restricts any other assembling of equivalent digital libraries in the scope of its specified domain. We provide mathematical foundation and reasoning of the proposed formulation, and confirm its feasibility and accountability in several case studies.