TECHNOLOGY FOCUS - ENVIRONMENT - Preferred Method: In the method (M1), pH of the activated sludge is 10-14 in the alkali treatment step, by adding an alkaline liquid to the activated sludge. In the method (M1), alkali treatment is conducted under at least one condition of the following (1) to (3): 1) holding the pH of the activated sludge greater than or equal to 13 for greater than or equal to 5 minutes, 2) holding the pH of the activated sludge 12-13 for greater than or equal to 10 minutes, and 3) holding the pH of the activated sludge 10-12 for greater than or equal to 60 minutes. In the method (M2), nitrite which has been formed in the step of oxidizing the ammonium nitrogen is denitrified by anaerobic ammonium oxidizing bacteria while using the ammonium nitrogen contained in the waste water as a hydrogen donor, in the step of subjecting the nitrite to the denitrification treatment. In the method (M2), the nitrite formed in the step of oxidizing the ammonium nitrogen is denitrified by denitrifying bacteria, in the step of subjecting the nitrite to the denitrification treatment. The method (M2) further involves: recovering the activated sludge used in the step of oxidizing the ammonium nitrogen to the nitrite; subjecting the recovered activated sludge to the alkali treatment; and adjusting the pH of the activated sludge which has been subjected to the alkali treatment, by adding an acid agent to the activated sludge. Preferred Apparatus: In the waste water treatment apparatus, the nitrite which has been formed in the nitrite-forming tank is denitrified by anaerobic ammonium oxidizing bacteria, while using the ammonium nitrogen contained in the waste water as a hydrogen donor, in the denitrification tank; and the nitrite which has been formed in the nitrite-forming tank is denitrified by denitrifying bacteria, in the denitrification tank. The waste water treatment apparatus further comprises: a recovery apparatus which recovers the activated sludge from the nitrite-forming tank; a regeneration apparatus which subjects the activated sludge which has been recovered by the recovery apparatus to the alkali treatment in it to regenerate the activated sludge; and a pH-adjusting apparatus which adjusts the pH of the activated sludge by adding an acid agent to the activated sludge which has been subjected to the alkali treatment. EXAMPLE - No suitable example is given.