PURPOSE: To reduce running cost by providing a device for neutralizing the gas from a reactor, device for capturing and separating fine particles of NH4Cl, etc., so that by-produced NH3 can be recycled at the time of descaling steel by gaseous NH4Cl.

CONSTITUTION: This gaseous ammonium chloride treatment device is formed of the reactor 1 for bringing the steel 1 and the NH4Cl vapor into contact with each other, the device 7 for cooling the gas therefrom and neutralizing the NH3 in the gas with HCl, a dust collector 13 and a carburettor 16. The dust collector 13 captures and separates the fine particulate NH4Cl contained in the gas from the neutralizing and cooling device 7. The carburettor 16 heats the NH4Cl separated by the dust collector 13 to form the vapor thereof and supplies the vapor to the reactor after controlling the vapor to the prescribed concn.