An erase method of a semiconductor device includes performing an operation comprised of supplying an erase pulse to erase the memory cells of a memory block, performing an erase verify operation for detecting memory cells of the memory block having threshold voltages dropped to a target erase voltage, from among the memory cells, performing a pre-program operation on the memory cells having the threshold voltages dropped to the target erase voltage, if, as a result of the erase verify operation, the memory block comprises memory cells having the threshold voltages higher than the target erase voltage and the memory cells having the threshold voltages dropped to the target erase voltage, and repeating the operation of supplying an erase pulse, the erase verify operation, and the pre-program operation until the threshold voltages of all the memory cells drop to the target erase voltage.