Flash memory device reducing noise of common source line, program verify method thereof, and memory system including that

PURPOSE: A flash memory device, a program verification method thereof, and a memory system including the same are provided to prevent the expansion of the width of a threshold voltage distribution due to the noise by reducing the noise of a common source line. CONSTITUTION: A plurality of memory cells(110) are connected between a bit line and a common source line. A data IO circuit(120) is connected to the bit line and stores the data to be programmed in a memory cell selected among a plurality of memory cells. The data IO circuit maintains the data to be programmed in a program verification operation and precharges the bit line according to the data to be programmed selectively.