Water treatment method and water treatment device

The water treatment method is for treating water to be treated that contains organic matter and a nitrogen component, and includes, in a biological treatment device including at least an aerobic tank: a biological treating for biologically treating inflowing water to be treated aerobically by using sludge that contains granules; a solid-liquid separating for separating the biological treatment solution in the aerobic tank into treated water and concentrated sludge using a solid-liquid separation device; and a sludge returning for returning the concentrated sludge obtained in the solid-liquid separating to a stage before the biological treatment device. The concentration of ammonium nitrogen remaining due to the nitrification reaction in the aerobic tank is measured with an ammonium nitrogen concentration measurement device, and the amount of gas containing oxygen dispersed to the aerobic tank is controlled with a blower so that the measured ammonium nitrogen concentration is within a prescribed range.