The purpose of this research was to clarify the causal relationship between both behavioral and psychological symptoms of dementia (BPSD) and psychosocial factors (stress state). Cognitive function and BPSD were evaluated using the Revised Hasegawa’s Dementia Scale and Neuropsychiatric Inventory, respectively, and stress was measured by the autonomic nervous activity and heart rate variability analysis. Using observations and latent variables, we constructed a causal hypothesis model that aggregated the causal relationship between cognitive function and stress state in one system. This model was analyzed using structural equation modeling. The results showed that cognitive function had a strong influence on autonomic nervous activity via BPSD; however, autonomic nervous activity had little influence on BPSD.