A fuel-mixing prevention filler neck device is provided. The fuel-mixing prevention filler neck device includes a fuel dispensing gun insertion unit that receives a fuel dispensing thereinto and a coupling unit that provides fluid communication with a bottom of the fuel dispensing gun insertion unit. The coupling unit is disposed on an exterior circumferential surface thereof with coupling protrusions for fastening with a plurality of slots formed in a filler pipe. Further, a first side of a lower portion of the coupling unit is open to form a coupling region. A misfueling prevention unit slidably moves to the coupling region to be coupled to the coupling unit. Additionally, the misfueling prevention unit is closed when a gasoline fuel dispensing gun is inserted and being opened when a diesel fuel dispensing gun is inserted.