To efficiently remove nitrate nitrogen in low-BOD wastewater containing nitrate nitrogen without increasing BOD value.

In a method for removing nitrate nitrogen in wastewater, by making low-BOD wastewater containing nitrite and nitrite a denitrification bacteria immobilized carrier prepared by immobilizing denitrification bacteria on a biodegradable immobilized carrier which is insoluble or poorly soluble in water under anaerobic atmosphere, an organic carbon source is supplied to the denitrification bacteria by the decomposition of the immobilized carrier, and the denitrification bacteria are propagated on the surface of the immobilized carrier, so that nitrate and/or nitrite contained in the wastewater is reduced into nitrogen gas.