A fuel tank system comprising: a fuel tank capable of storing fuel that is supplied to an engine; a pipe that communicates a gas layer at an upper portion inside the fuel tank with the external atmosphere; a deformation location that is provided at a portion of an upper wall portion of the fuel tank a deformation unit that deforms the deformation location such that the deformation location indents towards the inside of the fuel tank; a valve that is provided at the pipe, that opens the pipe to communicate the gas layer with the external atmosphere, and that closes off the pipe to seal the fuel tank; and a control unit that actuates the deformation unit with the valve in an open state after the gas layer has been communicated with the external atmosphere, and that closes the valve in a state in which the deformation location is indented towards the inside of the fuel tank, characterized in that the deformation location is provided at a periphery of a fuel pump provided at a central portion inside the fuel tank in plan view and that is capable of deforming so as to indent towards the inside of the fuel tank; wherein the deformation unit is a negative pressure pump that lowers the pressure inside the fuel tank by suctioning from the gas layer and is set so as to drive the negative pressure pump immediately after a lid has been closed after refueling, and wherein configuration is effected such that the deformation location returns to its original position on an extension line of the upper wall portion according to the increase in the pressure when the pressure inside the fuel tank increases to a specific value or greater from a state in which the deformation location is indented towards the inside of the fuel tank. a fuel tank capable of storing fuel that is supplied to an engine; a pipe that communicates a gas layer at an upper portion inside the fuel tank with the external atmosphere; a deformation location that is provided at a portion of an upper wall portion of the fuel tank a deformation unit that deforms the deformation location such that the deformation location indents towards the inside of the fuel tank; a valve that is provided at the pipe, that opens the pipe to communicate the gas layer with the external atmosphere, and that closes off the pipe to seal the fuel tank; and a control unit that actuates the deformation unit with the valve in an open state after the gas layer has been communicated with the external atmosphere, and that closes the valve in a state in which the deformation location is indented towards the inside of the fuel tank, characterized in that the deformation location is provided at a periphery of a fuel pump provided at a central portion inside the fuel tank in plan view and that is capable of deforming so as to indent towards the inside of the fuel tank; wherein the deformation unit is a negative pressure pump that lowers the pressure inside the fuel tank by suctioning from the gas layer and is set so as to drive the negative pressure pump immediately after a lid has been closed after refueling, and wherein configuration is effected such that the deformation location returns to its original position on an extension line of the upper wall portion according to the increase in the pressure when the pressure inside the fuel tank increases to a specific value or greater from a state in which the deformation location is indented towards the inside of the fuel tank.