A system connecting a printer and a host computer includes a signal processing unit, a printing unit, a DC/DC converter converting a power supply voltage into a lower voltage required by the signal processing unit and outputting the converted voltage to the signal processing unit, an input unit receiving a printing command, and a power supply controlling device. The power supply controlling device includes a first switching unit, a pulse width modulating unit, a power supply switch providing a power on/off signal from a user, and a power supply controlling unit outputting a control signal to the pulse width modulating unit to control the first switching unit to perform an on/off operation corresponding to the power on/off signal from the user.