A method for detecting a position of a signal source in a living body includes: arranging three electrodes on a surface of the living body and alternately connecting a first external resistance and a second external resistance in parallel between the electrodes and a ground potential; measuring first voltages Vi (i=1, 2, 3) generated at the respective electrodes when the first external resistance is connected in parallel between the electrodes and the ground potential, and second voltages Vi (i=1, 2, 3) generated at the respective electrodes when the second external resistance is connected in parallel between the electrodes and the ground potential; and calculating three ratios Vi/V′i (i=1, 2, 3) from the first and second voltages Vi and V′i, and detecting the position of the signal source in the living body based on the three ratios Vi/V′i (i=1, 2, 3).