Filler neck device with means against incorrect fuelling

The device (1) has a movable shutter (6) that moves between positions in which a passage (2a) of a filler neck (2) of a fuel tank of a diesel motor vehicle, is partially obscured and is free, respectively. Activating units have V shaped blocking parts (4c) movable between blocking and unblocking positions of the shutter. The units have contact parts (3) that are arranged in the neck, so as to separate the contact parts from a distance equal to reference minimum dimension. The blocking parts are in the unblocking position, to authorize the movement of the shutter towards the latter position.