To understand the eutrophication phenomena in Lake Biwa, Japan, a research project for the water quality formation processes was executed. This paper presents phosphorus mass changes in water body in Lake Biwa from April 1995 to January 2000 and discusses the main factors of the change. Phosphorus mass ranged from 190 to 350 t, with an average of 260 t. DP mass in 1995, '97 and '98 showed seasonal change patterns, while non-periodical phenomena seemed to control the PP mass. Phosphorus mass was increased about 15 t in epilimnion from April to July while nitrogen showed a 50 t decrease. This disagreement suggests that some processes selectively accumulated phosphorus in epilimnion during this period. However, such an increase in phosphorus was not observed in 1996 and '99. Air temperature was lower and much snowfall was recorded in 1996 and '99, suggesting that coldness in winter season weakened the phosphorus accumulation process to depress the mass in epilimnion in summertime. Water temperature increase seemed to be delayed in proportion to total snowfall, but thermocline was formed independently of coldness. Hence, the idea was abandoned that the delay of thermocline formation encouraged the sedimentation process, and biological activities were considered to be the main factor controlling phosphorus mass in the epilimnion in Lake Biwa.