A misfuelling prevention device for preventing inappropriate fuel from being added to a vehicle utilizes a diameter difference between a gasoline gun and a diesel gun in order to prevent misfuelling. The misfuelling preventing device is constructed such that when the diesel gun having a relatively larger diameter is inserted, a locking button is not pressed while a flapper is opened, whereas when the gasoline gun having a relatively smaller diameter is inserted, the locking button is pressed while the flapper is not opened. As a result, misfuelling of the vehicle with inappropriate fuel is fundamentally prevented.