1. An information processing device comprising a Central Processing Unit (CPU) for document collections having tags permitting semantic class identification appended to each document, the information processing device comprising: a search unit of the CPU that creates a plurality of semantic class units containing two, or more semantic classes based on a taxonomy that identifies relationships between semantic classes among a plurality of semantic classes, such that there is no semantic class in a semantic class unit which is an ancestor or a descendant of any other semantic class in the same semantic class unit; a frequency calculation unit of the CPU that, for each of the semantic class units, identifies documents that match that semantic class unit in the document collections, and the frequency calculation unit calculates a first frequency and a second frequency, the first frequency representing occurrence of the semantic class units in a first subset of documents of the document collections, and the second frequency representing occurrence of the semantic class units in a second subset of documents of the document collections; and a retrieval unit of the CPU that carries out a search in the document collections based on externally entered search terms, and separates the document collections into the first subset by determining that documents of the first subset comprise the search terms and into the second subset by determining that documents of the second subset do not comprise the search terms, wherein: the search unit is configured to identify the semantic class units and output the semantic class units to an enumeration tree in response to determining that the first frequency is higher than a first threshold value and that the second frequency is lower than a second threshold value; the taxonomy identifies relationships between semantic classes among the plurality of semantic classes in a hierarchical manner; and as the search unit carries out a child class conversion process and a class addition process in the enumeration tree whose root is a null set, the search unit creates the enumeration tree by designating one, two or more of the semantic classes as nodes and establishing links between the nodes, and extracts a combination of semantic classes in which the semantic classes are not an ancestor or a descendant of each other to create the semantic class units.