1. A method for extracting a meaningful frequent itemset from a database in which a plurality of records each containing an itemset consisting of one or more items is stored by calculation processing of a computer, the plurality of items in the database having an order in which the items are arranged, the method comprising the steps of: (a) if there exist one or more items after a last item in an order of items in an old itemset, wherein the old itemset is an immediately preceding investigation target, deciding, by the computer, a new itemset that is a new investigation target by adding one of the one or more items after the last item to the old itemset in descending order, wherein the old itemset has an initial value of an empty item φ being the first item in the order; (b) registering, by the computer, a frequency of occurrence of the new itemset in a table prepared on a memory of the computer on the condition that the frequency of occurrence of the new itemset is equal to or more than a predetermined threshold; (c) in response to the registration in the table, determining, by the computer, using a value in the table, whether there is a correlation between each item in the new itemset and a subset of remaining items and in which that item is removed from the new itemset, wherein the correlation is determined if each item in the new itemset occurs in cooperation with the occurrence of the remaining items exclusive of that item, and wherein the determination of correlation is made by determination of whether an expression |T|×T(Y)|≧α×|T(Y−{y})|×|T({y})| is satisfied, where |T| denotes a total number of records, |T(Y)| denotes a frequency of occurrence of a new itemset Y, |T({y})| denotes a frequency of occurrence of any item y in the new itemset Y, |T(Y−{y})| denotes a frequency of occurrence of a subset of remaining items in which that item y is removed from the new itemset Y, and α denotes a predetermined threshold larger than one; (d) registering, by the computer, the new itemset in a set of the meaningful frequent itemsets on the condition that the determination is positive for each of all the items in the new itemset; and (e) in response to the registration in the set of the meaningful frequent itemsets, by the computer, setting the new itemset to the old itemset and repeating the step (a) before deciding a new itemset on the basis of the original old itemset.