FUEL TANK SYSTEM

A fuel tank system is obtained in which the internal pressure of a fuel tank does not readily increase due for example to a rise in temperature, and there is also no need to secure a space for deformation at the periphery of the fuel tank. A vapor pipe (34) is connected to a fuel tank (14), and an atmosphere release pipe (24) is connected through a canister (18). An electromagnetic valve (52) is attached to the vapor pipe, communicating a gas layer (A) at an upper portion inside the fuel tank with the external atmosphere by opening the electromagnetic valve. A thinned deformation location (14D) capable of indentation-deformation towards the inside is provided at an upper wall portion (14C) of the fuel tank. A negative pressure pump (58) is provided at the atmosphere release pipe 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.