A brain activity estimation device comprising a brain activity estimation means, the brain activity estimation means comprising: a blood-circulation-amount calculating unit configured to calculate time-series blood-circulation-amount data on a facial surface of a human based on RGB data of photographed image data on the facial surface acquired in time series, the photographed image data including an image of at least the forehead and/or paranasal sinus peripheral region of the facial surface, the RGB data being obtained by conducting RGB processing on the photographed image data, the RGB processing being to decompose the photographed image data into three color components composed of an R component, a G component and a B component; an analysis unit configured to decompose the blood-circulation-amount data by singular value decomposition, principal component analysis, or independent component analysis to obtain a plurality of components, and plot a time distribution and a space distribution of each of the components in graphs to make a blood-circulation-amount distribution diagram of the component; and an estimation unit configured to determine brain activity of the human, characterized in that: the analysis unit is further configured to determine whether or not each component satisfies a predetermined condition that there is a change in the blood-circulation-amount at the forehead and/or paranasal sinus peripheral region with regard to the component based on the blood-circulation-amount distribution diagram, and extract the component that satisfies the predetermined condition as a determination component among the plurality of components; and the estimation unit is configured to determine the brain activity of the human based on the determination component. a blood-circulation-amount calculating unit configured to calculate time-series blood-circulation-amount data on a facial surface of a human based on RGB data of photographed image data on the facial surface acquired in time series, the photographed image data including an image of at least the forehead and/or paranasal sinus peripheral region of the facial surface, the RGB data being obtained by conducting RGB processing on the photographed image data, the RGB processing being to decompose the photographed image data into three color components composed of an R component, a G component and a B component; the photographed image data including an image of at least the forehead and/or paranasal sinus peripheral region of the facial surface, the RGB data being obtained by conducting RGB processing on the photographed image data, the RGB processing being to decompose the photographed image data into three color components composed of an R component, a G component and a B component; an analysis unit configured to decompose the blood-circulation-amount data by singular value decomposition, principal component analysis, or independent component analysis to obtain a plurality of components, and plot a time distribution and a space distribution of each of the components in graphs to make a blood-circulation-amount distribution diagram of the component; and decompose the blood-circulation-amount data by singular value decomposition, principal component analysis, or independent component analysis to obtain a plurality of components, and plot a time distribution and a space distribution of each of the components in graphs to make a blood-circulation-amount distribution diagram of the component; and an estimation unit configured to determine brain activity of the human, characterized in that: the analysis unit is further configured to determine whether or not each component satisfies a predetermined condition that there is a change in the blood-circulation-amount at the forehead and/or paranasal sinus peripheral region with regard to the component based on the blood-circulation-amount distribution diagram, and extract the component that satisfies the predetermined condition as a determination component among the plurality of components; and the estimation unit is configured to determine the brain activity of the human based on the determination component. the analysis unit is further configured to determine whether or not each component satisfies a predetermined condition that there is a change in the blood-circulation-amount at the forehead and/or paranasal sinus peripheral region with regard to the component based on the blood-circulation-amount distribution diagram, and extract the component that satisfies the predetermined condition as a determination component among the plurality of components; and determine whether or not each component satisfies a predetermined condition that there is a change in the blood-circulation-amount at the forehead and/or paranasal sinus peripheral region with regard to the component based on the blood-circulation-amount distribution diagram, and extract the component that satisfies the predetermined condition as a determination component among the plurality of components; and the estimation unit is configured to determine the brain activity of the human based on the determination component.