A fuel tank system is obtained, in which internal pressure does not readily increase and in which there is no need to secure a space for deformation at the periphery of the fuel tank. A vapor pipe is connected to a fuel tank, and an atmosphere release pipe is connected through a canister. An electromagnetic valve is attached to the vapor pipe, communicating a gas layer at an upper portion inside the fuel tank with the external atmosphere by opening the electromagnetic valve. A thinned deformation location capable of indentation-deformation towards the inside is provided at an upper wall portion of the fuel tank. A negative pressure pump is provided to reduce the pressure inside the fuel tank, causing the deformation location to indentation-deform towards the inside. The fuel tank is sealed by closing the electromagnetic valve in a state in which the deformation location is indentation-deformed towards the inside.