Various attacks against RC5 have been analyzed intensively. A known plaintext attack has not been reported that it works on so higher round as a chosen plaintext attack, but it can work more efficiently and practically. In this paper, we investigate a known plaintext attack against RC5 by improving a correlation attack. As for a known plaintext attack against RC5, the best known result is a linear cryptanalysis. They have reported that RC5-32 with 10 rounds can be broken by 264 plaintexts under the heuristic assumption: RC5-32 with r rounds can be broken with a success probability of 90% by using 26r+4 plaintexts. However, their assumption seems to be highly optimistic. Our known plaintext correlation attack can break RC5-32 with 10 rounds (20 half-rounds) in a more strict sense with a success probability of 90% by using 263.67 plaintexts. Furthermore, our attack can break RC5-32 with 21 half-rounds in a success probability of 30% by using 263.07 plaintexts.