An automatic fluid-supply apparatus for a boiler system automatically controls an amount of fluid being supplied to a fluid tank of the boiler system. The automatic fluid-supply apparatus is provided with a flux detecting portion which comprises a rotor assembly disposed inside a fluid-supply pipe to rotate in proportion to the flux of the fluid flowing from a fluid-supply source to the fluid tank and a HALL sensor disposed close to the rotor assembly at an outer surface of the fluid tank. The HALL sensor is intermittently turned ON/OFF in response to permanent magnets provided to the rotor assembly. A controller compares the ON/OFF frequency with a preset ON/OFF frequency range and transmits an ON/OFF signal to a solenoid valve.