A power source includes a voltage setting unit configured to set an output voltage, a voltage generation unit configured to output the set voltage to a load, a feedback unit configured to detect the output voltage and feed back the detected voltage to the voltage setting unit, a current detection unit configured to detect a current value which is a sum of a current value flowing in the feedback unit and a current value flowing in the load when the set voltage is output to the load, and a control unit configured to switch between a constant current control which controls the set voltage so that the detected current value becomes a constant current, and a constant voltage control which controls the set voltage so that the voltage output to the load becomes a constant voltage based on the voltage value that is fed back by the feedback unit.