The increment of opportunities for using machine learning (ML) technologies has brought a new threat to cryptosystems. As a remarkable example, the ML technologies have gradually been employed in the side-channel attack (SCA) to obtain sensitive information. In this paper, the authors focus on the structure of a masked S-Box in AES, which aims to equip the SCA resistance even for the attacks using the ML technologies. More precisely, this paper analyzes the mathematical structure of the inverse operation over F(24)2 which is an isomorphic field for obtaining efficient arithmetic for the AES, so that all functions in the encryption scheme can handle masked data as it is. The mathematical structure is realized by introducing several mathematical tools such as the Gauss periods and the Itoh-Tsujii inversion algorithm, and as a result, we clarified the factors of the coefficients of A-1 for an element A F(24)2. It enables us to generate the corresponding element directly, which allows canceling the mask even after processing the SubBytes.