A stylus pen that includes a tip electrode, a power supply circuit, a signal detector, and a drive circuit where an output signal is transmitted to and an input signal is received from a touch panel via the tip electrode. The power supply circuit of the stylus pen selectively supplies a first supply voltage or a second supply voltage. The signal detector of the stylus pen detects an enable signal from the input signal and the signal detector of the stylus pen controls the power supply circuit to supply the first supply voltage in response to presence of the enable signal. The drive circuit operates at the first supply voltage to generate the output signal.