1. A method for finding maximal frequent itemsets over data streams including continuously generated transactions, the method including: when the itemsets included in previously generated transactions and a frequency of the itemsets are managed by a prefix tree and each of the nodes of the prefix tree has information, such as the appearance frequency of the itemsets corresponding to the nodes in question, a maximum lifetime, which is a maximum point in time that may allow the itemsets to remain in a frequent state even when no itemset appears later, a mark indicating whether the itemsets are the maximal frequent itemsets, or the like, (a) receiving transaction T k generated at a current point in time; (b) updating the information owned by each node corresponding to the itemsets included in the transaction T k among the nodes of the prefix tree; (c) adding each node corresponding to the itemsets, which are not managed in the prefix tree among the itemsets included in the transaction T k , to the prefix tree and setting the information on the added nodes; and (d) finding the maximal frequent itemsets by checking whether the corresponding itemsets are frequent while visiting ones of the nodes of the prefix tree that allows the mark to indicate the maximal frequent itemsets.