Deposition of seston, chlorophyll-a, and nutrients in Lake Chuzenji, which is oligotrophic lake with 163m maximum depth, were observed every one month for two years. The annual amounts of deposition at 110m depth were obtained to be 1241, 0.582, 52.6, 5.83, 1.27g/m²y for seston, chlorophylly-a, carbon, nitrogen, and phosphorus, respectively. On the basis of the content of Ti, the separation of deposited matter into allochthonous and autochthonous matter was examined. The decomposition rates during sedimentation and at sediments were estimated from those data. The significance of deposition of particulate matter was discussed with the observed input fluxes. Particularly, those phenomena expained the low value of dissolved oxygen near sediments and the increace of NO₃-N in lower layer in stratified period. The average values of sinking rate were calculated to be 2.5m/d for seston, and 0.67, 1.19m/d for chlorophyll-a at the depths of 30m and 110m, respectively. In addition to the cross correlation of those rates for each sampling period at each depth, the content of chlorophyll-a in seston effects the sinking rates of seston and chlorophyll-a