2. The device according to claim 1 wherein said representative-to-target code converter calculates the scores according to the factors comprising: .vertline.W.sub.i .vertline.: Number of characters of character stream W.sub.i, i=1 to n; Prob(W.sub.i): Frequency of W.sub.i to exist in articles; Prob(POS.sub.i .vertline.POS.sub.i-1): Probability of a term with POS (part-of speech) of W.sub.i to follow a term with POS of W.sub.i-1 in articles; and C.sub.i : Number of characters in the picked-up character stream that are different from its corresponding character in the original character stream.
2. The device according to claim 1 wherein said representative-to-target code converter calculates the scores according to the factors comprising: .vertline.W.sub.i .vertline.: Number of characters of character stream W.sub.i, i=1 to n; Prob(W.sub.i): Frequency of W.sub.i to exist in articles; Prob(POS.sub.i .vertline.POS.sub.i-1): Probability of a term with POS (part-of speech) of W.sub.i to follow a term with POS of W.sub.i-1 in articles; and C.sub.i : Number of characters in the picked-up character stream that are different from its corresponding character in the original character stream. 3. The device according to claim 2 wherein said representative-to-target code converter calculates the scores according to the following equation: ##EQU7##