A body activity measurement device is provided with a light irradiation unit that irradiates near infrared light of a specified wavelength on a specified measurement site of a brain of a subject; a light detector that detects the intensity of the light exiting from the measurement site; a light absorbance data generator that from the time sequence changes of the intensity of the exiting light generates light absorbance time sequence data suggesting the changes in the concentration of oxyHb, deoxyHb and totalHb; and multiple baseline correction units that correct baselines that mutually differ in relation to the light absorbance time sequence data obtained when the subject performs specified assignments; and configured such that the baseline correction of the light absorbance time sequence data can be conducted using the baseline correction units corresponding to the assignment performed by the subject.