A brain activity feedback system, comprising: a brain activity measurement device configured to optically measure brain activity of a subject, which includes a first measurement channel and a plurality of second measurement channels herein, the first measurement channel is configured to acquire measurement data by a light transmitting probe and a light receiving probe which, in use, are arranged in a cephalic portion of the subject with a predetermined first distance, and each of the plurality of second measurement channels is configured to acquire the scalp blood flow data by a light transmitting probe (la) and a light receiving probe which, in use, are arranged in the cephalic portion of the subject with a second distance less than the first distance; a data processing unit configured to calculate a statistic representing statistical significance of a change in the measurement data, which reflects the brain activity, on the basis of the measurement data; and a display unit feeding back a brain activity index corresponding to the statistic to the subject by displaying the brain activity index, in real time, wherein the data processing unit is configured to calculate the statistic by correcting a scalp blood flow component in the measurement data of the first measurement channel on the basis of the scalp blood flow data of the plurality of second measurement channels, in real time, and characterised in that the data processing unit is configured to calculate a first main component obtained by performing main component analysis with respect to the scalp blood flow data of the plurality of second measurement channels in each predetermined time range in real time, as correction amount of the scalp blood flow component. a brain activity measurement device configured to optically measure brain activity of a subject, which includes a first measurement channel and a plurality of second measurement channels herein, the first measurement channel is configured to acquire measurement data by a light transmitting probe and a light receiving probe which, in use, are arranged in a cephalic portion of the subject with a predetermined first distance, and each of the plurality of second measurement channels is configured to acquire the scalp blood flow data by a light transmitting probe (la) and a light receiving probe which, in use, are arranged in the cephalic portion of the subject with a second distance less than the first distance; a data processing unit configured to calculate a statistic representing statistical significance of a change in the measurement data, which reflects the brain activity, on the basis of the measurement data; and a display unit feeding back a brain activity index corresponding to the statistic to the subject by displaying the brain activity index, in real time, wherein the data processing unit is configured to calculate the statistic by correcting a scalp blood flow component in the measurement data of the first measurement channel on the basis of the scalp blood flow data of the plurality of second measurement channels, in real time, and characterised in that the data processing unit is configured to calculate a first main component obtained by performing main component analysis with respect to the scalp blood flow data of the plurality of second measurement channels in each predetermined time range in real time, as correction amount of the scalp blood flow component.