PURPOSE: To prevent the hazards such as explosion in water gas reaction, by diluting the feed air with inert gas, thereby reducing the oxygen concentration below a specific level.

CONSTITUTION: Air 4 is diluted 2-times or more with inert gas 10 to reduce the oxygen concentration to ≤10%, and introduced to the inner part of the partition plates 2 of the char layer 9 to form a combustion zone 5. The exhaust gas G1 produced in the combustion zone 5 is discharged from the reactor 1. On the other hand, the char heated at high temperature in the combustion zone 5 is transferred to the water gas reaction zone 6, where it is reacted with steam 8 to produce water gas.