The fuel tank system comprises a tank body for containing fuel therein, a canister for disposing said fuel component, a filler tube connected with the tank body and forming a channel for supplying fuel to the tank body, a vent side check valve having a first chamber and a second chamber separated by a diaphragm, a signal tube connecting the first chamber of the vent side check valve with the upper part of the filler tube, a vent side tube for connecting the tank body with the canister via the second chamber of the vent side check valve and for introducing a gaseous body containing fuel component in the tank body to the canister via the second chamber when filling fuel, and an orifice for the signal tube, provided on the upper part of the signal tube on the filler tube side and having such an inside diameter and a length that may prevent the fuel permeation into the signal tube side from the filler tube side. Therefore, by the functions of the orifice, the permeation of the fuel into the signal tube can be controlled thus to maintain the functions of the vent side check valve.