Disclosed is an apparatus for stimulating the brain and measuring the light induced neuronal activity including a signal application unit which applies a signal to a living tissue to stimulate the neuronal cells in the living tissue; an electrode unit which detects an electrophysiological signal of the neuronal cell in response to the signal; and an insulation unit which controls an impedance of the electrode unit. The signal application unit is formed integrally with the electrode unit, so that the site where the signal is applied to the living tissue is approximated to the site where the response to the stimulation is measured.