PURPOSE: To prevent environmental pollution by penetrating an NOx decreasing agent into a flue gas flow to reduce NOx at most by improving reaction rate, by supplying the NOx decreasing agent directly to the catalyst separately from pollutant-containing waste gas and without coming into contact with remaining heating surfaces (heating surfaces of a heat storage chamber).

CONSTITUTION: A pollutant-containing waste gas in heat exchange with another medium is treated by using a heat storage type heat exchanger 1 by supplying a pollutant decreasing agent from the hot side of a constant amount storage mass 3 formed at least partially of a catalytic material or a rotating storage mass 3. At the same time, the decreasing agent is supplied directly to the catalyst section 3, 4 separately from the pollutant-containing waste gas and without coming into contact with remaining heating surfaces (heating surfaces of a heat storage, chamber 4). Consequently, a reaction rate is improved, NOx is reduced at most, and the NOx decreasing agent does not pollute environments with no penetration into the flue gas stream.