In this paper, we propose a novel optical fault detection scheme for RSA hardware based on Montgomery exponentiation, which can effectively detect optical fault injection during the exponent calculation. To protect the RSA hardware from the optical fault injection attack, we implemented integrity check logic for memory and optical fault detection logic for Montgomery-based multiplier. The proposed scheme is considered to be safe from various type of attack and it can be implemented with no additional operation time and small area overhead which is less than 3%.