Anxiety disorders are characterized by excessive fear. Although cognitive-behavioral therapy is widely used for treating anxiety disorders, a substantial portion of the patients (> 40%) suffer from relapses. We previously showed that the infralimbic cortex (IL), a brain region which suppresses fear responses, plays a critical role in fear reinstatement, which represents the relapse of anxiety disorders. In this study, we sought to identify the cellular basis in the IL for fear reinstatement. We obtained whole-cell recordings from pyramidal neurons in layer 5 of the IL. Fear reinstatement was associated with a decrease in the frequency of miniature excitatory postsynaptic currents (mEPSCs) and an increase in the paired-pulse ratio (PPR) of evoked EPSCs. Importantly, the degree of fear reinstatement was correlated with these changes in the mEPSC frequency and the PPR. On the other hand, the intrinsic excitability of IL neurons was unchanged after fear reinstatement. We also found that blockade of dopamine D1/D5 receptors in the IL prevented the decrease in the mEPSC frequency as well as fear reinstatement. These results suggest that the decreased synaptic inputs in the IL may be a crucial component of fear reinstatement.