Abstract: The device comprises a hollow external body, operationally arranged in a filler of the tank and anchored in which, with the interpositioning of a permanent magnet, is an internal body which defines an essentially tubular passage. The arrangement is such that when the nozzle of a urea solution distributor pistol is inserted into the passage of the internal body, the magnet causes an opening of an on/off valve in the pistol, permitting the input of a flow of the said solution into the tank. The device also comprises a filter through which the urea solution is intended to pass. At its end turned operatively towards the outside of the tank the internal body is provided with suitable means of connection for snapping onto corresponding retention formations provided in the external body, and the filter is integrated with the end of the external body intended to be turned towards the inside of the tank.