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SHACAL-2 is a 256-bit block cipher with various key sizes based on the hash function SHA-2. Recently, it was recommended as one of the NESSIE selections. Up to now, no security flaws have been found in SHACAL-2. In this paper, we discuss the security of SHACAL-2 against an impossible differential attack. We propose two types of 14-round impossible characteristics and using them, we attack 30-round SHACAL-2 with 512-bit key. This attack requires 744 chosen plaintexts and has time complexity of 2^495.1 30-round SHACAL-2 encryptions.