The mass change of nitrogen in Lake Biwa was shown, and its cycle was discussed and compared with the input/output loading. Nitrogen exists at 10.5X10³ ton in average, and its minimum and maximum mass are 7.6X10³ ton, 13.4X10³ ton, respectively. Its average mass is equivalent to 133% of the annual input loading, resulting in 1.6 months of residence time, which is shorter than 5.5 years of hydraulic retention time. This indicates that nitrogen in the lake is removed by settling onto the lake bottom or denitrification, and the removed mass is estimated to be 71% of the input loading. Phytoplanktons mainly utilize ammonium nitrogen in epilimnion whose mass is only 1% of the total mass in the lake. rather than other inorganic nitrogen amounting to more than ten-fold of ammonium nitrogen. During the stratification period, almost all decomposed nitrogen in the organic matter is oxidized into nitrate, and half of them is likely to be removed by denitrification.