Abstract: A device for preventing backfire of inflammable gases comprising a cylinder, a balance spring, and a repelling magnet case. The cylinder holds both a valve casing whose rear edge shuts a gas induction passage when the casing is pushed backward by high pressure, and a check valve which contains a repelling magnet and which shuts and opens a gas supplying passage and a flow-way formed in the valve casing. The repelling magnet case, which is pushed backward by the balance spring, contains a repelling magnet aligned face-to-face with a magnet that is contained in the check valve.