Lake Biwa in Japan has a stable gyre during seasons of thermal stratification. A vertical circulation consisting of downwelling in the central area and upwelling near the rim of the gyre is calculated by the diagnostic model. The energetics in the model indicate that this vertical circulation corresponds to the decay process of the gyre. Tracking of drifters in summer supports the presence of a vertical circulation, though its magnitude is about 50% less than that estimated by the diagnostic model. This is explained from the ratio of kinetic energy and available potential energy of the gyre. The realistic value of the vertical eddy viscosity is estimated to be 0.8--2.1 cm2 s-1 in summer. Monthly variations of the horizontal and vertical circulations are calculated by the diagnostic model using the monthly mean density field. The gyre and the vertical circulation are maximal in September. The characteristic decay time scale of the gyre is estimated to be about 10 days. The effect of topographic differential heating is considered to be one of the maintenance mechanisms of the gyre in early summer, and the wind stress curl to be the important maintenance force for the gyre from late summer to late fall.