A fuel tank system comprising: a fuel tank for holding fuel; a canister capable of adsorbing fuel vapor inside the fuel tank and releasing the fuel vapor; a gas separator that separates air components from gases in the fuel tank and discharges the separated air components externally to the fuel tank; an internal pressure detecting means for detecting internal pressure of the fuel tank; and switching means, characterized in that the switching means comprises: a communication pipe with one end connected to the fuel tank, an intermediate section that branches at a branching section, and the other ends respectively connected to the canister and the gas separator; a discharge pipe for externally discharging air components from the gas separator; a three-way valve provided to the branching section of the communication pipe and capable of selectively switching a discharge path for gas from the fuel tank between a canister side communication path that communicates the fuel tank with the canister and a gas separator side communication path that communicates the fuel tank with the gas separator; and an open-shut valve provided at a location on a path from the fuel tank via the gas separator to the discharge pipe, wherein the switching means communicates the fuel tank and the canister at a time of supplying fuel to the fuel tank, and switches between a first state in which the internal pressure of the fuel tank is in a negative pressure state of a negative pressure of a first specific value or lower and in which fuel vapor that has been adsorbed by the canister is sucked into the fuel tank by the negative pressure, and a second state in which the internal pressure of the fuel tank is in a positive pressure state of a positive pressure of a second specific value or higher and in which gas of a gas layer is caused to pass through the gas separator and air components in the gas layer are caused to be externally discharged from the gas separator by the positive pressure, at a time of not supplying fuel to the fuel tank, wherein, in the first state, the switching means communicates the fuel tank and the canister, and in the second state, the switching means communicates the fuel tank and the gas separator. a fuel tank for holding fuel; a canister capable of adsorbing fuel vapor inside the fuel tank and releasing the fuel vapor; a gas separator that separates air components from gases in the fuel tank and discharges the separated air components externally to the fuel tank; an internal pressure detecting means for detecting internal pressure of the fuel tank; and switching means, a communication pipe with one end connected to the fuel tank, an intermediate section that branches at a branching section, and the other ends respectively connected to the canister and the gas separator; a discharge pipe for externally discharging air components from the gas separator; a three-way valve provided to the branching section of the communication pipe and capable of selectively switching a discharge path for gas from the fuel tank between a canister side communication path that communicates the fuel tank with the canister and a gas separator side communication path that communicates the fuel tank with the gas separator; and an open-shut valve provided at a location on a path from the fuel tank via the gas separator to the discharge pipe, wherein the switching means communicates the fuel tank and the canister at a time of supplying fuel to the fuel tank, and switches between a first state in which the internal pressure of the fuel tank is in a negative pressure state of a negative pressure of a first specific value or lower and in which fuel vapor that has been adsorbed by the canister is sucked into the fuel tank by the negative pressure, and a second state in which the internal pressure of the fuel tank is in a positive pressure state of a positive pressure of a second specific value or higher and in which gas of a gas layer is caused to pass through the gas separator and air components in the gas layer are caused to be externally discharged from the gas separator by the positive pressure, at a time of not supplying fuel to the fuel tank, wherein, in the first state, the switching means communicates the fuel tank and the canister, and in the second state, the switching means communicates the fuel tank and the gas separator.