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.

Multi Fluid Injection Mixer for injecting gas and/or liquid as admixture fluid to gas and/or liquid flowing through a pipe, and homogeneously mixing the admixture fluids and pipe fluids, said injection mixer constituting a section of the pipe, distinguished in that the injection mixer is comprising: at least one contacting element having at least one contacting surface facing and deflecting some of the pipe fluid flow, forming a constriction to the internal cross-section of the pipe, such that the pipe fluid flow is accelerated and fluid flowing in the vicinity of said surface is deflected to flow along the surface until the surface end over a sharp edge at the point of maximum constriction and flow velocity, at least one injection element arranged with a fluid connection to said surface of the contacting element, such that admixture fluid can be injected onto said surface and along said surface be entrained by the flowing pipe fluid over the sharp edge, but for a contacting element formed as a contracting pipe section at least two injection elements are provided.