The objective of this study was modeling of causal relationship between water quality and spectral reflectance. In this paper, linearity between water quality (chlorophyll-a and suspended solid) and spectral reflectance were firstly evaluated to choose non-linear transformation of the water quality variables. Next, the direct influences from each water quality to spectral reflectance ware evaluated by partial correlation analysis. Finally, the modeling of causal relationship between water quality and spectral reflectance was performed by using path diagram for Lake Kasumigaura with latent variables possibly corresponding to SS quality and quantity.