To establish a repowering system for an old coal fired boiler having high economy which is characteristic of a coal gasification system and having less formation of NOx in combustion gas.

A coal gasification device 9 is supplemented to an existing coal fired boiler 6 to collect dry distillation gas and char generated in the device 9 with a char collector 11, followed by separation thereof to use a portion of the dry distillation gas as a fuel for a gas turbine 12 and using the remaining dry distillation gas and char as a fuel for the boiler 6 for combustion. With such a system, despite the necessity of oil or gas fuel at the time of starting operation, only coal can be used in a normal operation. Since a portion of the dry distillation gas can be used as fuel for the existing coal fired boiler, a strong reduction region can be formed in a boiler furnace. Moreover, NOx formation in a boiler furnace which is a problem in a normal coal fired boiler, can be considerably reduced owing to the denitrification effect in the furnace by the dry distillation gas.