A valve device for a fuel tank, comprising: a case having an air flow valve orifice communicated with outside of the tank at an upper part thereof, and a fuel inflow part below the air flow valve orifice, said case being divided into an upper chamber and a lower chamber by a partition formed between the air flow valve orifice and the inflow part; an upper float body disposed inside the upper chamber of the case, and ascending by fuel flowing into the case to be seated in said air flow valve orifice; and a lower float body having an upwardly projecting guide shaft, being supported inside the lower chamber of the case with the guide shaft being inserted to be movable vertically inside an insertion part formed on said partition, and ascending by the fuel flowing into the case to close a main connect-through part between the lower chamber and the upper chamber, formed on said partition, wherein said upper float body has a receiving hole to receive the guide shaft of the lower float body, wherein said guide shaft comprises a head part on an upper end thereof, and a coupling part for passing from below the head part is formed on an upper end of the insertion part by elastic deformation, characterized in that at least a portion of the coupling part of the insertion part comprises an arm piece projecting sideways from an upper end of an upwardly projecting base part piece to couple a projecting end with the head part, wherein a disk-form body having an outer diameter roughly equal to a hole diameter of a through-hole is placed directly above a through-hole of a dividing wall, such that the outer edge thereof overlaps on the hole edge of the through-hole and such that a gap is opened in the vertical direction between the lower face of the disk-form body and the upper face of the dividing wall, wherein bridge pieces spanning between the disk-form body and the dividing wall are provided in four places opening spaces in the direction around the center of the disk-form body, such that the disk-form body is supported on the dividing wall by these bridge pieces in four places. a case having an air flow valve orifice communicated with outside of the tank at an upper part thereof, and a fuel inflow part below the air flow valve orifice, said case being divided into an upper chamber and a lower chamber by a partition formed between the air flow valve orifice and the inflow part; an upper float body disposed inside the upper chamber of the case, and ascending by fuel flowing into the case to be seated in said air flow valve orifice; and a lower float body having an upwardly projecting guide shaft, being supported inside the lower chamber of the case with the guide shaft being inserted to be movable vertically inside an insertion part formed on said partition, and ascending by the fuel flowing into the case to close a main connect-through part between the lower chamber and the upper chamber, formed on said partition, wherein said upper float body has a receiving hole to receive the guide shaft of the lower float body, wherein said guide shaft comprises a head part on an upper end thereof, and a coupling part for passing from below the head part is formed on an upper end of the insertion part by elastic deformation, characterized in that at least a portion of the coupling part of the insertion part comprises an arm piece projecting sideways from an upper end of an upwardly projecting base part piece to couple a projecting end with the head part, wherein a disk-form body having an outer diameter roughly equal to a hole diameter of a through-hole is placed directly above a through-hole of a dividing wall, such that the outer edge thereof overlaps on the hole edge of the through-hole and such that a gap is opened in the vertical direction between the lower face of the disk-form body and the upper face of the dividing wall, wherein bridge pieces spanning between the disk-form body and the dividing wall are provided in four places opening spaces in the direction around the center of the disk-form body, such that the disk-form body is supported on the dividing wall by these bridge pieces in four places.