An apparatus for preventing backflow of a fill valve in a water toilet. The apparatus includes a fill valve for permitting and stopping supply of washing water from a hydrant into a water tank, a fixing section fixing the fill valve to an overflow pipe, a float having a packing for opening or closing an opening/closing hole of the fill valve depending on a level of water contained in the water tank, and a backflow-preventing section disposed inside the fill valve. The backflow-preventing section permits washing water to be supplied from the hydrant into the water tank, and prevents washing water from flowing back toward the hydrant. The backflow-preventing section includes a backflow-preventing member mounted to a supply pipe of the fill valve, and an operation control valve seated on a fitting protrusion of the fill valve to control an operation of the backflow-preventing member.