This paper proposes an effective method for reducing test data volume under multiple scan chain designs. The proposed method is based on reduction of distinct scan vectors using selective dont-care identification. Selective dont-care identification is repeatedly executed under condition that each bit of frequent scan vectors is fixed to binary values (0 or 1). Besides, a code extension technique is adopted for improving compression efficiency with keeping decompressor circuits simple in the manner that the code length for infrequent scan vectors is designed as double of that for frequent ones. The effectiveness of the proposed method is shown through experiments for ISCAS89 and ITC99 benchmark circuits.