Abstract: A downdraft gasifier has a gas producer. The gas producer includes an introducing portion provided in an upper portion for introducing solid biomass fuel 4, an air inlet 17 provided in a central portion, a fire grate 14 provided in a lower portion, and a product gas delivery portion provided below the fire grate. In this gasifier, there are provided fire-proofball laid layers in which a plurality of metal balls 15 are disposed in layers on the fire grate substantially uniform, and a preheating unit (such as a burner) for preheating the fire-proof balls. The metal balls 15 are preheated in advance to a predetermined temperature ranging from 500° C. to 800° C. before the solid biomass fuel is introduced into the producer. After that, the solid biomass fuel 4 is introduced into the producer, and gas produced in the producer is delivered from a portion below the fire grate.