A gas concentration control device for pressure vessel is provided for controlling gas concentration inside an accommodation space that is set in an interior of a chamber formed inside a pressure vessel curing oven. The pressure vessel curing oven includes a gas inlet tube with a first valve and a gas outlet tube with a second valve, which are in communication with the accommodation space. The gas concentration control device includes a gas concentration detection device, which is operative for timed detection of concentration of a gas inside the accommodation space, and a control unit for setting a predetermined concentration, performing a judgment if the gas concentration inside the accommodation space reaches the predetermined concentration according to a detection result of the gas concentration detection device, and thereby controls the first valve and the second valve according to result of the judgment.