An adsorption/desorption device is communicated with an upper space of a fuel tank via a vapor passage. The adsorption/desorption device is configured to selectively adsorb or desorb air components contained in vapor. The adsorption/desorption device and the vapor passage are not communicated to the external atmosphere. The fuel tank is provided with a pressure detecting device for detecting the internal pressure of the fuel tank. A pressure regulating device is provided to the vapor passage for controlling and maintaining the pressure applied to the adsorption/desorption device. When the internal pressure of the fuel tank is higher than the atmospheric pressure, the internal pressure of the fuel tank is pressure-fed to the side of the adsorption/desorption device via the pressure regulating device until the internal pressure of the fuel tank becomes in equilibrium with the atmospheric pressure. When the internal pressure of the fuel tank is lower than the atmospheric pressure, vapor is pressure-fed to the side of the fuel tank via the pressure regulating device until the internal pressure of the fuel tank becomes in equilibrium with the atmospheric pressure.