To provide a waste gas neutralization device and a method for operating the same by which scale is prevented from being stuck on a region where scale is easily stuck, the long-term continuous operation is possible and labor and cost for maintenance are saved.

The head part of a waste gas blowing pipe installed in a gas- liquid reaction tank 6 is obliquely cut, and a fresh water injection pipe equipped with a spray nozzle is installed within the waste gas blowing pipe to jet fresh water to scale stuck on the head part of the waste gas blowing pipe. A three- way valve 16 is installed at a junction of sludge piping 14 to prevent sludge from accumulating in the piping. Furthermore, a fixed quantity of waste smoke washing liquid is fed to a fly ash dissolving tank 3, and waste liquid of the same quantity as has been increased when the water level of a fly ash service tank 12 is raised is fed to a first dehydrator 15a or a second dehydrator 15b.