To improve the efficiency of a boiler and reduce frequency for operation stopping of a boiler for cleaning.

This combustion system has a furnace chamber 12 having four walls, plural black liquor injecting guns provided on at least two walls of the furnace chamber 12 and injecting black liquor drops into the furnace, plural primary air ports 30 provided on low places of the furnace, an injecting positional combustion zone 50 provided on surrounds of the black liquor injecting guns and plural quaternary air ports 34 provided in the injecting positional combustion zone 50 or its upper part of the wall in the furnace. Then, a kraft recovering boiler furnace wherein air injected from the quaternary air port 34 without pulling in a solid fuel from the black liquor is introduced into the injecting positional combustion zone 50 and reacts with a gas fuel discharged from the black liquor is obtained.