There are provided a nitrogen treating method and system for a nitrogen compound, which can treat the nitrogen compound efficiently and which can reduce the size and cost of an apparatus. The gist is that, in a nitrogen treating method wherein a nitrogen compound in for-treatment water is treated according to an electrochemical technique, a cathode reaction region and an anode reaction region are defined by a cation exchange membrane interposed between a cathode and an anode, and the for-treatment water treated in the cathode reaction region according to the electrochemical technique is treated with hypohalogenous acid, or, ozone or active oxygen according to a chemical technique.