A response period is determined arbitrarily. A satisfactory detecting ability may not be provided because of an artifact or the like contained in a signal. Therefore, an effective detection method and an effective display method of presenting results of detection must be developed. A series of tasks (stimuli) or a selected measurement signal is used as a reference signal, and a phase difference of any other measurement signal from the reference signal is calculated. The synchronousness of the phase of the measurement signal with the phase of the reference signal is numerically expressed. The thus obtained numerical value is statistically processed in order to numerically express a degree of reliability. Thus, a brain activity or a functional connectivity is visualized.