A hermetically sealed fuel tank system includes a hermetically sealed fuel tank, a fuel pump configured to supply fuel from within the hermetically sealed fuel tank to an internal combustion engine, and a pressure regulating valve. The pressure regulating valve is disposed within the hermetically sealed fuel tank and is configured to regulate the fuel pressure of the fuel supplied from the fuel pump to the internal combustion engine. The pressure regulating valve includes a pressure regulating chamber and a backpressure chamber partitioned by a diaphragm. An ambient air introduction passage is fluidly connected to the backpressure chamber and is configured to introduce air from the atmosphere outside the fuel tank into the backpressure chamber.