Provided is a flash memory device. The flash memory device includes: a memory cell storing multi-bit data; a reference bias voltage supply circuit generating a reference bias voltage; an sense amplifier sensing the multi-bit data stored in the memory cell using the reference bias voltage; and a control circuit controlling the reference bias voltage supply circuit. The control circuit controls the reference bias voltage supply circuit to allow the reference bias voltage to be developed according to a change of a main word line voltage applied to the memory cell during a read operation.