A power switch device provides and cuts off power supply to a load in which a MOSFET for causing a direct current power source to be connected and disconnected is provided between power supply lines and the direct current power source, located upstream of the power supply lines, the power supply lines including a ripple capacitor for stabilizing the supply voltage against fluctuations in the load current in the load, powered by the direct current power source. The power switch device is operated to transition power supply from opening to closing by gradually increasing the output voltage of the MOSFET to minimize the value of current charging the ripple capacitor.