An active stylus pen is insensitive to external noise and has enhanced sensing performance with respect to an uplink signal input from a touch screen. The active stylus pen includes a housing connected to a ground, a conductive tip protruding outwardly from one side of the housing and brought into contact with a touch screen, a conductor layer surrounding an outer surface of the housing with an insulator interposed therebetween, a pen driving circuit connected to the conductor layer, and a switch connecting the conductor layer and the pen driving circuit. The pen driving circuit is cased by the housing, receives an uplink signal and a touch sensor driving signal from the touch screen, generates a pen driving signal synchronized with the touch sensor driving signal and outputs the generated pen driving signal to the touch screen through the conductive tip.