Almost all of the current public-key cryptosystems (PKCs) are based on number theory, such as the integer factoring problem and the discrete logarithm problem (which will be solved in polynomial-time after the emergence of quantum computers). While the McEliece PKC is based on another theory, i.e. coding theory, it is vulnerable against several practical attacks. In this paper, we carefully review currently known attacks to the McEliece PKC, and then point out that, without any decryption oracles or any partial knowledge on the plaintext of the challenge ciphertext, no polynomial-time algorithm is known for inverting the McEliece PKC whose parameters are carefully chosen. Under the assumption that this inverting problem is hard, we propose slightly modified versions of McEliece PKC that can be proven, in the random oracle model, to be semantically secure against adaptive chosen-ciphertext attacks. Our conversions can achieve the reduction of the redundant data down to 1/3 ∼ 1/4 compared with the generic conversions for practical parameters.