A brain activity estimation device includes a facial skin temperature acquisition element and a brain activity estimation element. The acquisition element detects a skin temperature of at least part of a human face, and acquires facial skin temperature data including detected temperature data and location data of a detection region in a time series manner. The estimation element estimates human brain activity based on the facial skin temperature data acquired. The estimation element decomposes the facial skin temperature data into a plurality of components, extracts a component as a determination component, and determines whether the determination component is related to the human brain activity based on a presence or absence of a temperature change in a prescribed region of the human face.