Hippocampal adult neurogenesis is hypothesized to contribute spatial pattern separation. However, it is unclear whether this effect depends on the structure. In order to find structure independent functions at a cell population level, we aimed at understanding how adult neurogenesis promotes spatial pattern separation on a scale of a group of cells. Hippocampal neuronal networks with or without immature neurons were trained with two types of electrical stimulations with different spatial patterns by microelectrode array. Ratio of response times of trained stimuli to the other decreased only in the network with immature neurons, suggesting that the immature neurons promote spatial pattern learning. The result indicates that adult neurogenesis enhances the topological learning of spatial patterns.