A decline in the function of the brain as a result of diseases such as dementia is accurately and quantitatively evaluated by a simple method using signals acquired by a small number of sensors disposed on the scalp. The present invention is a brain activity measurement device provided with: a signal acquisition means that acquires signals from the brain using sensors attached at three different locations on the surface of the head of an examinee； a data extraction means that extracts from the signals acquired by the respective sensors a signal of a specific frequency band arising from the activity of a deep part of the brain and that extracts data from the extracted signal at a sampling period； a correlation value calculation means that calculates based on the phase relationship of three pieces of time series data extracted by the data extraction means for each sensor a correlation value indicating a correlation relationship of the signals acquired by the respective sensors； and an index value calculation means that calculates based on the calculated correlation value an index value for determining the brain function by analyzing the signal from the deep part of the brain.
