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This paper presents the developed open-vocabulary spoken document retrieval system including the newly proposed subphonetic segment(SPS) unit and combining multilayer subword units. There are two principal approaches to the task of Spoken Document Retrieval(SDR), the subword-based approach and the word-based approach. An inevitable problem of this approach is the fact that the vocabulary size is limited. An alternative approach is to perform retrieval by subword-based transcriptions resulted from a subword recognizer. Subword-based SDR has the advantages that the recognizer is less expensive and open-vocabulary retrieval is possible, because the recognition component is not bound to any vocabulary. Our approach to SDR is based on a subword recognizer which initially transforms the spoken documents into subword sequences. From the experimental evaluation on the Japanese retrieval we confirmed that using the proposed SPS unit and the combination of multilayer subword units is effective for open-vocabulary spoken document retrieval.