A semiconductor device and an operating method thereof comprise peripheral circuits configured to apply an erase voltage to memory cells when performing an erase operation, and sense a voltage change of bit lines by an erase verification voltage applied to word lines of the memory cells when performing an erase verification operation to thereby detect cells which are not erased, and a control circuit configured to control the peripheral circuits by changing a sensing reference level for determining the voltage change of the bit lines when the cells which are not erased are detected when performing the erase verification operation, so that the erase verification operation is repeatedly performed.