PURPOSE: To obtain an engine in which emission of NOx, CO and UHC is reduced while suppressing damage due to backfire.

CONSTITUTION: In order to reduce emission of nitrogen oxides, carbon monoxide and unburnt hydrocarbon, flame temperature must be kept between 1371°C (2500°F) and 1538°C (2800°F). Fuel and air are mixed using a premixing tube and mixture gas is combusted in a combustion chamber 7. Since air/fuel mixture gas passes through premixing tubes 8, 10, 12, 16, 18, 20, the premixing tubes have anti-backfire properties. When a load acting on a combustor is required to decrease significantly lower than 100% of maximum load during off-peak time, for example, low emission and low flame temperature can be sustained even if the load is reduced by regulating the flow rate of fuel and air.