A valve device for a fuel tank, comprising: a case having a vent valve port arranged at an upper portion of the case and connectable to an outside of the tank, and an inflow portion for fuel arranged below the vent valve port; a float element accommodated in the case to move vertically and having a valve element provided with a seal member at an upper portion thereof, the float element raising to seat the seal member on the vent valve port when the fuel flows into the case through the inflow portion; a ventilation portion to connect between inside of the tank and inside of the case, formed at the upper portion of the case and positioned above a waterline of the float element when the float element is seated, wherein an amount of ventilation through the ventilation portion when the float element is seated is reduced by the upper portion of the float element being positioned in closer proximity to the upper portion of the case; and a regulation portion arranged at the upper portion of the case to keep an interval constant between the upper portion of the case and the upper portion of the float element when the float element is seated, characterized in that the valve element is provided with a short cylindrical base that has a cylinder top end closed by a top plate provided with a through hole at its center and that has its cylinder bottom end opened, and with the seal member which is made of rubber or synthetic resin possessing rubber-like elasticity and covers part of a face portion at the top plate that forms part of the upper portion of the float element wherein a portion of the face portion that is not covered by the seal member is positioned in closer proximity to the opening of through holes that are formed at the upper portion of the case and that form the ventilation portion, whereby the amount of ventilation through the ventilation portion is reduced. a case having a vent valve port arranged at an upper portion of the case and connectable to an outside of the tank, and an inflow portion for fuel arranged below the vent valve port; a float element accommodated in the case to move vertically and having a valve element provided with a seal member at an upper portion thereof, the float element raising to seat the seal member on the vent valve port when the fuel flows into the case through the inflow portion; a ventilation portion to connect between inside of the tank and inside of the case, formed at the upper portion of the case and positioned above a waterline of the float element when the float element is seated, wherein an amount of ventilation through the ventilation portion when the float element is seated is reduced by the upper portion of the float element being positioned in closer proximity to the upper portion of the case; and a regulation portion arranged at the upper portion of the case to keep an interval constant between the upper portion of the case and the upper portion of the float element when the float element is seated, characterized in that the valve element is provided with a short cylindrical base that has a cylinder top end closed by a top plate provided with a through hole at its center and that has its cylinder bottom end opened, and with the seal member which is made of rubber or synthetic resin possessing rubber-like elasticity and covers part of a face portion at the top plate that forms part of the upper portion of the float element wherein a portion of the face portion that is not covered by the seal member is positioned in closer proximity to the opening of through holes that are formed at the upper portion of the case and that form the ventilation portion, whereby the amount of ventilation through the ventilation portion is reduced.