Filling device intended to be installed on a toilet tank and to be coupled to water supply means, the device comprising a tap intended to be coupled to water supply means and the tank having a wall provided with an orifice dedicated to the passage of the tap and extending between an inner face and an outer face of the tank, the tap comprising: - a main body intended to extend in height inside the tank, the main body bearing means for distributing water and means for opening/closing water circulation between said supply means and the distribution means, - a connecting end intended to extend outside the reservoir, the main body being telescopic and comprising a male part and a female part movable in height on the male part, the male part having an upper end open inside the female part, the female part extending along a longitudinal axis X between an open lower end and a closed upper end in the vicinity of which the female part carries the water distribution means and the water circulation opening/closing means, the female part has a water path communicating with the distribution means, an annular seal ensuring sealing between the male part and the female part, the water path being secant to the longitudinal axis X, characterised in that and the annular seal is carried by the female part, so as to maintain a constant height H between a lower part of the annular seal and a low point of the distribution means, to prevent a suction of water for re-injecting it into pipes of a building, while allowing a height adjustment of the tap in order to create, when the tank is full, below the lowest point of the outlet port, a minimum air supply. - a main body intended to extend in height inside the tank, the main body bearing means for distributing water and means for opening/closing water circulation between said supply means and the distribution means, - a connecting end intended to extend outside the reservoir, the main body being telescopic and comprising a male part and a female part movable in height on the male part, the male part having an upper end open inside the female part, the female part extending along a longitudinal axis X between an open lower end and a closed upper end in the vicinity of which the female part carries the water distribution means and the water circulation opening/closing means, the female part has a water path communicating with the distribution means, an annular seal ensuring sealing between the male part and the female part, the water path being secant to the longitudinal axis X, characterised in that and the annular seal is carried by the female part, so as to maintain a constant height H between a lower part of the annular seal and a low point of the distribution means, to prevent a suction of water for re-injecting it into pipes of a building, while allowing a height adjustment of the tap in order to create, when the tank is full, below the lowest point of the outlet port, a minimum air supply. the main body being telescopic and comprising a male part and a female part movable in height on the male part, the male part having an upper end open inside the female part, the female part extending along a longitudinal axis X between an open lower end and a closed upper end in the vicinity of which the female part carries the water distribution means and the water circulation opening/closing means, the female part has a water path communicating with the distribution means, an annular seal ensuring sealing between the male part and the female part, the water path being secant to the longitudinal axis X, characterised in that and the annular seal is carried by the female part, so as to maintain a constant height H between a lower part of the annular seal and a low point of the distribution means, to prevent a suction of water for re-injecting it into pipes of a building, while allowing a height adjustment of the tap in order to create, when the tank is full, below the lowest point of the outlet port, a minimum air supply.