Abstract: A vent passage communicated to an upper space of a fuel tank mounted on an automobile is connected to a canister. A two-way valve, which includes a pair of one-way valves disposed oppositely and in parallel to each other, is provided in the vent passage. One one-way valve is adjusted its valve opening pressure in a two-staged manner according to the operational state of an internal combustion engine. In another method, a bypass passage bypassing the two-way valve is provided in the vent passage. A solenoid valve and a one-way valve are interposed in the bypass passage. The above one one-way valve of the two-way valve is set at a first valve opening set-up pressure, and the other one-way valve is set at a second valve opening set-up pressure lower than the first valve opening set-up pressure. The solenoid valve is opened or closed according to the operational state of the internal combustion engine.