The nitrogen reduction reaction (NRR), which produces NH3 by reducing N2 under ambient conditions, is attracting attention as a promising technology that can reduce energy consumption in industrial processes. We investigated the adsorption behaviors at various active sites on the Ni (100) surface, which is widely used among catalytic metal surfaces capable of adsorbing and activating N2, based on density-functional theory calculations. We also investigated two N2 adsorption structures, so-called end-on and side-on structures. We find that for the end-on case, N2 is adsorbed on a top site, and the reaction proceeded in a distal pathway, while for the side-on case, N2 is adsorbed on a bridge site, and the reaction proceeded with enzymatic pathway. Finally, this study provides insight into the adsorption of metal catalyst surfaces for the NRR reactions based on the calculated Gibbs free energy profiles of the thermodynamically most favorable pathways.