1. A data search device comprising: a data storage unit configured to store a plurality of data groups into which pieces of search target data are classified and a plurality of decryption keys for decrypting encrypted identifiers of the data groups, by associating each of the plurality of the data groups with each of the plurality of the decryption keys; a key-determination information storage unit configured to store key-determination information which is a value for determining a decryption key; a search condition input unit configured to obtain a search condition which is a condition for searching for data, and obtain encrypted information generated by encrypting an identifier of a data group to be searched; a data group selecting unit configured to decrypt the encrypted information obtained by the search condition input unit by using one or more of the plurality of the decryption keys stored in the data storage unit, thereby generating one or more corresponding decryption results, identify a decryption key of the plurality of the decryption keys that corresponds to a matched decryption result among the one or more corresponding decryption results, the matched decryption result being the same as the key-determination information, and select as a search target group a data group associated with the identified decryption key, from among the plurality of the data groups stored in the data storage unit; a data searching unit configured to obtain as search result data a piece of data that satisfies the search condition obtained by the search condition input unit, from among pieces of data included in the search target group selected by the data group selecting unit; and a search result output unit configured to output the search result data obtained by the data searching unit.