A gas production system which is configured to apply plasma to a catalyst in a reactor and reform a supplied source gas (8, 23) and a supplied oxidant gas to produce a product gas (14, 18, 24, 25), the gas production system comprising: - source gas supply means for supplying the source gas (8, 23) to the reactor; - oxidant gas supply means for supplying the oxidant gas to the reactor; - gas ratio change means (101, 102) for changing a ratio between a supply amount of the source gas (8, 23) to be supplied to the reactor by the source gas supply means and a supply amount of the oxidant gas to be supplied to the reactor by the oxidant gas supply means, to make a state in which the oxidant gas is abundant as compared to the source gas (8, 23); and plasma generation means (3, 4, 15, 17) for generating the plasma to be applied to the catalyst, the gas production system further comprising: - a gas production device (1, 16) having the reactor, a first electrode and a second electrode for generating the plasma therebetween, and a catalyst layer provided in the reactor and containing the catalyst; and - an external power supply which is connected to the first electrode and the second electrode and generates voltage (15, 17), wherein the plasma is generated in the reactor by the voltage generated by the external power supply (15, 17) and applied between the first electrode and the second electrode, wherein the catalyst layer is provided in the space between the first electrode and the second electrode. the gas production system comprising: - source gas supply means for supplying the source gas (8, 23) to the reactor; - oxidant gas supply means for supplying the oxidant gas to the reactor; - gas ratio change means (101, 102) for changing a ratio between a supply amount of the source gas (8, 23) to be supplied to the reactor by the source gas supply means and a supply amount of the oxidant gas to be supplied to the reactor by the oxidant gas supply means, to make a state in which the oxidant gas is abundant as compared to the source gas (8, 23); and plasma generation means (3, 4, 15, 17) for generating the plasma to be applied to the catalyst, - source gas supply means for supplying the source gas (8, 23) to the reactor; - oxidant gas supply means for supplying the oxidant gas to the reactor; - gas ratio change means (101, 102) for changing a ratio between a supply amount of the source gas (8, 23) to be supplied to the reactor by the source gas supply means and a supply amount of the oxidant gas to be supplied to the reactor by the oxidant gas supply means, to make a state in which the oxidant gas is abundant as compared to the source gas (8, 23); and plasma generation means (3, 4, 15, 17) for generating the plasma to be applied to the catalyst, the gas production system further comprising: - a gas production device (1, 16) having the reactor, a first electrode and a second electrode for generating the plasma therebetween, and a catalyst layer provided in the reactor and containing the catalyst; and - an external power supply which is connected to the first electrode and the second electrode and generates voltage (15, 17), - a gas production device (1, 16) having the reactor, a first electrode and a second electrode for generating the plasma therebetween, and a catalyst layer provided in the reactor and containing the catalyst; and - an external power supply which is connected to the first electrode and the second electrode and generates voltage (15, 17), wherein the plasma is generated in the reactor by the voltage generated by the external power supply (15, 17) and applied between the first electrode and the second electrode, wherein the catalyst layer is provided in the space between the first electrode and the second electrode.