A wastewater treatment device treats nitrogen wastewater containing hydrogen peroxide by using micro/nano bubbles generated in a micro/nano bubble reaction tank. Thereby, activity of microbes is increased in next denitrification tank and nitrification tank, so that microbial treatment efficiency can be increased and the denitrification tank and the nitrification tank can be downsized. Thus, the wastewater treatment device makes it possible to enhance treatment efficiency of nitrogen wastewater containing hydrogen peroxide and to reduce initial and running costs for wastewater treatment.