In this study, to understand the mechanism of human gait, we applied the 2-dimensional gait to a human musculoskeletal model with 70 muscles, and investigated the extension method to 3-dimensional gait. We tested 4 optimization conditions to achieve a 14 degrees of freedom 3-dimensional gait using a musculoskeletal model with 70 muscles, and verified whether the model could achieve gait or not. As a result, gait was achieved only under condition 4, in which the DOFs were added step by step in the frontal axis, vertical axis, and then in the sagittal axis at each joint. The optimization procedure for condition 4 resulted in a 5-second gait. However, the results differed from those of the subject experiments in terms of stride length, average gait speed, and ankle joint angular change, and we discussed points that should be considered in the future. In the future, we would like to reproduce 3-dimensional gait, taking into consideration the points mentioned in the discussion, and contribute to the understanding of the mechanism of human gait.