Scan-based attacks are techniques to decipher a secret key using scanned data obtained from a cryptography circuit. Public-key cryptography, such as RSA and elliptic curve cryptosystem (ECC), is extensively used but conventional scan-based attacks cannot be applied to it, because it has a complicated algorithm as well as a complicated architecture. This paper proposes a scan-based attack which enables us to decipher a secret key in ECC. The proposed method is based on detecting intermediate values calculated in ECC. By monitoring the 1-bit sequence in the scan path, we can find out the register position specific to the intermediate value in it and we can know whether this intermediate value is calculated or not in the target ECC circuit. By using several intermediate values, we can decipher a secret key. The experimental results demonstrate that a secret key in a practical ECC circuit can be deciphered using 29 points over the elliptic curve E within 40 seconds.