In a gas supply system of one embodiment, a gas control ECU performs an initial monitoring step of comparing first detection information of a high-pressure sensor to a first threshold value and, after it is determined that the first detection information has become less than or equal to the first threshold value, performs a secondary monitoring step of comparing second detection information of a mid-pressure sensor to a second threshold value. The gas control ECU causes a valve-open period and a valve-closed period of an injector in the secondary monitoring step to be longer than the valve-open period and the valve-closed period of the injector in the initial monitoring step.