A gas mixing device capable of safely mixing flammable gas containing, for example, methane or the like and combustion supporting gas such as oxygen-containing gas, and a synthesis gas producing device using this gas mixing device. Flammable gas containing methane or the like and combustion supporting gas such as oxygen-containing gas are supplied into a mixing vessel via a first gas supplying section and a second gas supplying section respectively, and these gases are mixed within a combustion range in the vessel to be discharged via a discharge section. In the mixing vessel, packings for forming a large number of narrow gas flow passages in the vessel are packed so that velocity of the mixed gas flowing in the vessel becomes higher than burning velocity of the flammable gas and the combustion supporting gas.