Chronic pain is a debilitating condition that often emerges as a clinical symptom of inflammatory diseases. It has therefore been widely accepted that the immune system critically contributes to the pathology of chronic pain. Microglia, a type of immune cell in the central nervous system, has attracted researchers' attention because in rodent models of neuropathic pain that develop strong mechanical and thermal hypersensitivity, histologically activated microglia are seen in the dorsal horn of spinal cord. Several kinds of cytokines are generated by damaged peripheral neurons and contribute to microglial activation at the distal site of the injury where damaged neurons send their projections. Microglia are known as key players in the surveillance of the local environment in the central nervous system and have a significant role of circuit remodeling by physical contact to synapses. Key molecules for the pathology of neuropathic pain exist in the activated microglia, but the factors driving pain-inducible microglial activation remain unclear. Therefore, to find the key molecules inducing activation of spinal microglia and to figure out the precise mechanism of how microglia modulate neuronal circuits in the spinal cord to form chronic pain state is a critical step for developing effective treatment of neuropathic pain.