Enhancing security level against potential internal eavesdroppers in Non-Orthogonal Multiple Access (NOMA) has been reported as a serious challenge. In this letter, a novel secure NOMA scheme based on a secure downlink transmission strategy is proposed to address this challenge. The proposed scheme is designed to provide a high level of security against potential internal eavesdroppers while maintaining a high data rate. The security performance of the proposed scheme is evaluated through simulations, and the results show that the proposed scheme can provide a significant improvement in security compared to existing schemes.