A catalytic reactor including: a reaction container filled with a catalyst having ruthenium; and an introduction part for introducing a material fluid to the reaction container. The reaction container is supplied with a mixed gas as material fluid to produce synthetic gas containing hydrogen and carbon monoxide in the presence of the catalyst. The introduction part is connected to the reaction container in a horizontal direction or a direction inclined downward from above a horizontal plane, and a height of the catalyst filled in the reaction container is adjusted above a height at which the material fluid is fed from the introduction part to the reaction container. Also, a method of using the catalytic reactor including monitoring a molar ratio of carbon and oxygen contained in the material fluid and stopping the supply of an oxygen-containing gas to the reaction container before the molar ratio becomes excessively low.