1. A pattern matching method for matching between a first symbol sequence of m symbols and a second symbol sequence of n symbols, the method comprising the steps of: for each i (1<=i<=m) and j (1<=j<=n), comparing i-th symbol s(i) of the first symbol sequence with j-th symbol t(j) of the second symbol sequence, and setting a state stat(i, j) which indicates a state of a node of i and j node(i, j) according to the comparison, wherein if s(i) matches to t(j), then node(i, j) is set to a value that indicates correct, otherwise node(i, j) is set to a value that indicates incorrect; for each a, b, and c (1<=a<a+c<=m, 1<=b<b+c<=n), counting the maximum number of consecutive nodes pc in which all states of stat(a, b), stat(a+1, b+1),. .. , stat(a+c, b+c) indicate correct; and determining similarity of the first and second symbol sequences based on the maximum number pc.