Glial cells are vital for the modulation of synaptic connections and healthy development of brain networks. They control the excitatory / inhibitory synaptic balance and assemble neural circuity by synapse-formation through synaptogenic factors or by synapse-elimination through phagocytosis. We have recently revealed that astrocytes form excitatory synapses in the adult injured brain, for which mGluR5 has a pivotal role. However, astrocytic mGluR5 is, in health brain, expressed in the limited time-window of the postnatal developmental stage (critical period). All these findings suggest that astrocytic mGluR5 has a crucial role in generation of synapses in the critical period. Thus, we surveyed how astrocytic mGluR5 destines the subsequent synapse scaling using astrocyte-specific mGluR5 KO mice (cKO). Astrocytes in cKO were slightly reactive in the critical period. Unexpectedly, the number of excitatory synapses did not decrease much, instead, the number of inhibitory synapses decreased significantly in cKO throughout ages. This decrease was due to phagocytosis by microglia. In fact, microglia frequently engulfed inhibitory synaptic elements in the critical period in cKO. Hence, we conclude that astrocytes organize inhibitory network in the critical period by somehow modulating microglial phagocytic activity, for which mGluR5 has an inhibitory role. 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.