PURPOSE: To decrease COD substances such as a dithioic acid etc., in simultaneous removal process of SOx and NOx in flue gases, by blowing ozone into the absorption liquid stored at the bottom of an absorption tower through a nozzle, and at the same time spraying said liquid in said tower.

CONSTITUTION: An air feed pipe 16 extending from an air blower 9 is connected to an air-intake port of an ozonizer 15, while an air flow controller valve 17 is connected to said air feed pipe 16. An ozone feed pipe 19 having spray nozzles 18 on its forward end is connected to an ozone delivery port of the ozonizer 15, while the spray nozzles 18 are led into the absorption liquid 6 at the bottom of an absorption tower 1 so that ozone can be fed into said liquid 6. Further, a flow meter 20 to measure the flow rate of flue gases flowing through a pipe 7 and a concentration meter 21 to measure the NOx concentration in the flue gases are provided on the midway of the feed gas pipe 7 leading to the absorption tower 1. Based on detected signals from these meters, feed quantities of air to the ozonizer are changed.