A brain activity estimation device (10) is provided with a facial skin temperature acquisition means (20) and a brain activity estimation means (30). The facial skin temperature acquisition means (20) detects skin temperature in at least part of a human face, and acquires facial skin temperature data in a time series manner. The facial skin temperature data includes detected temperature data and location data of the detection region thereof. On the basis of the facial skin temperature data acquired by the facial skin temperature acquisition means (20), the brain activity estimation means (30) estimates the human brain activity. The brain activity estimation means (30) decomposes the facial skin temperature data into a plurality of components by singular value decomposition, principal component analysis, or independent component analysis. The brain activity estimation means (30) extracts a component as a determination component from among the plurality of components. The component has the amplitude of the component waveform thereof is correlated to a change at a time when the brain is at rest and a time when the brain is stimulated. The brain activity estimation means (30) determines whether the determination component is related to the human brain activity on the basis of the presence or absence of a temperature change in a prescribed region of a human face.