The prefrontal cortex (PFC) is known to play a pivotal role in maintaining the upcoming task information without the sensory input. To examine how the prefrontal microcircuits implement active memory maintenance, we used calcium imaging in mice performing a delayed Go/No-Go task. We found that the dorsomedial PFC (dmPFC) pyramidal neurons exhibited robust delay activity, with different subpopulations representing Go and No-Go action plans. Inhibiting pyramidal neurons through the optogenetic activation of somatostatin (SST)- or parvalbumin (PV)-positive interneurons, even transiently during the delay period, significantly impaired task performance. In marked contrast, activating vasoactive intestinal peptide (VIP)-positive interneurons enhanced the behavioral performance and increased memory representation in pyramidal neurons, partly by inhibiting SST or PV neurons. SST/VIP neuron activation also impaired/enhanced performance of a delayed two-alternative forced choice task, indicating that the microcircuit mechanism can be generalized to a different memory-guided behavior. These results suggest that dmPFC is a critical component of the working memory machinery and that VIP neurons can dynamically regulate the functional gain of pyramidal neurons in the dmPFC.