Test compression/decompression schemes using variable-length coding, e.g., Huffman coding, efficiently reduce the test application time and the size of the storage on an LSI tester. In this paper, we propose a model of an adaptive decompressor for variable-length coding and discuss its property. By using a buffer, the decompressor can operate at any input and output speed without a synchronizing feedback mechanism between an ATE and the decompressor, i.e., the proposed decompressor model can adapt to any test environment. Moreover, we propose a method for reducing the size of the buffer embedded in the decompressor. Since the buffer size depends on the order in which test vectors are input, reordering test vectors can reduce the buffer size. The proposed algorithm is based on fluctuations in buffered data for each test vector. Experimental results show a case in which the ordering algorithm reduced the size of the buffer by 97%.