To efficiently catch acidic gas such as HCl gas and SOx contained in exhaust gas generated at time of incineration of waste and simultaneously to stabilize harmful heavy metal contained in fly ashes which are obtained by collecting soot and dust generated from exhaust gas.

This processing agent contains at least one kind selected from the group consisting of a neutralizer which contains slaked lime and magnesium hydroxide as a main component and furthermore contains different kinds of cement and aluminum hydroxide or the like, an inorganic adsorbent such as aluminum silicate and silicon dioxide, a chelating agent such as dimethyldithiocarbamate, water glass, phosphate such as water-soluble sodium phosphate and a compound which reacts with a harmful metallic compound such as lead and produces a slightly soluble or insoluble compound. The processing agent is blown into an exhaust gas treatment process to treat exhaust gas and thereafter soot and dust produced from exhaust gas are collected by a dust collector or the like. Collected fly ashes are kneaded by adding water in accordance with necessity.