Glial cells are vital for the modulation of synaptic connections and healthy brain development. They control the excitatory / inhibitory synaptic balance and assemble neural circuity by synaptic formation or elimination. We have recently revealed that astrocytes form excitatory synapses in the adult injured brain through mGluR5 signaling. However, astrocytic mGluR5 is, in health brain, expressed in the limited time-window of the postnatal developmental stage. Thus, we surveyed whether / how astrocytic mGluR5 destines the subsequent synaptic assembly using astrocyte-specific mGluR5 KO mice (cKO). Unexpectedly, the number of excitatory synapses did not alter much in cKO, instead, the number of inhibitory synapses decreased significantly in cKO throughout ages. Interestingly, microglia frequently engulfed inhibitory synaptic elements in cKO. Astrocytic mGluR5 expression was transient event in the critical period, however, behavioral dysfunction was observed even in adult cKO mice. Next, we surveyed the astrocytic molecule which regulates microglial engulfment. We focused on astrocytic IL7, which is decreased in cKO, as a candidate for such molecules. IL7 treatment decreased engulfment-related gene expression in microglia. Hence, we conclude that astrocytes organize inhibitory network in the critical period by modulating microglial phagocytic activity. It should be noted that although mGluR5 is only transiently expressed in astrocytes in the critical period, its function greatly affects the inhibitory neuronal networks throughout life.