We analyze the security of the SEED block cipher against differential attacks. SEED is a 16-round Feistel cipher developed by the Korea Information Security Agency. The SEED proposers estimated their cipher against differential cryptanalysis in a self-estimation document and found a six-round differential characteristic with probability 2-130. We present an improved method of examining the differential characteristics of SEED and show three six-round differential characteristics with probability 2^-124. These characteristics allow us to attack sevenround SEED, which surpasses the proposers estimation. Our differential attack needs 2¹²⁶ chosen-plaintext pairs and 2¹²⁶ computations of the F function to deduce the subkey used in the last round of seven-round SEED.