1. A speech retrieval apparatus for searching, in a speech database, for an audio file matching an input search term, comprising: a first conversion device configured to convert the audio file in the speech database into an acoustic model serialization code, a phonemic code, a sub-word unit, and a speech recognition result; a first division device configured to divide the acoustic model serialization code, the phonemic code, the sub-word unit, and the speech recognition result converted by the first conversion device; a first speech retrieval unit creation device configured to create a first speech retrieval unit by using the acoustic model serialization code, the phonemic code, the sub-word unit, and the speech recognition result divided by the first division device as elements; a second conversion device configured to convert the input search term into an acoustic model serialization code, a phonemic code, a sub-word unit, and a speech recognition result; a second division device configured to divide the acoustic model serialization code, the phonemic code, the sub-word unit, and the speech recognition result converted by the second conversion device; a second speech retrieval unit creation device configured to create a second speech retrieval unit by using the acoustic model serialization code, the phonemic code, the sub-word unit, and the speech recognition result divided by the second division device as elements; and a matching device configured to match the first speech retrieval unit and the second speech retrieval unit so as to determine a matching degree between the input search term and the audio file, and determine a matching result according to the matching degree, wherein each of the acoustic model serialization codes includes searchable text obtained by serializing mel-frequency cepstrum coefficients with a vector quantization technique.