1. A method for finding frequent itemsets using a compressed prefix tree in a restricted memory space, the method comprising steps of: updating parameters about finding the frequent itemsets comprising a total number of transactions when a new transaction appears; updating a frequency count of an itemset that appears in the new transaction about a transaction monitored by the compressed prefix tree through searching the compressed prefix tree; inserting or pruning the itemsets in the compressed prefix tree among the itemsets not monitored by the compressed prefix tree considering a support and a length of the itemset; merging or splitting nodes corresponding to respective itemsets which are monitored using the undated frequency count in the compressed prefix tree; and finding frequent itemsets among itemsets monitored by the compressed prefix tree.