Various embodiments of the invention relate generally to an electronic device, and more particularly, to a semiconductor memory device and an operating method thereof. A semiconductor memory device including a plurality of memory cells using an optimal input buffer reference voltage may include at least one input buffer receiving data to be stored in the plurality of memory cells, and an input buffer reference voltage control unit setting one of a plurality of internal voltages generated beforehand and having different voltage levels as a reference voltage of the at least one input buffer in response to a control signal received from a controller controlling the semiconductor memory device.