This paper proposes a co-channel interference cancellation method for multiple-input multiple-output (MIMO) wireless communication systems. Maximum-likelihood multi-user detection (ML-MUD), which is one of the co-channel interference cancellation methods at a receiver side, has excellent bit error rate (BER) performance. However, computational complexity of the ML-MUD is prohibitive, because the ML-MUD must search for the most probable symbol vector from all candidates of the transmitted signals. We apply sphere decoding (SD) to the ML-MUD in order to reduce the computational complexity of the ML-MUD, and moreover we propose a modified version of the SD suitable for the ML-MUD. The proposed method extracts desired signal components from a received signal vector and a channel matrix decomposed the upper triangular form, and then performs the SD to the low dimensional model in order to detect the transmitted signals of the desired user. Computer simulation confirms that the proposed method can suppress the undesired signals and detect the desired signals, offering significant reduction of the computational complexity of the conventional method.