In an in-vivo signal source detection method, three electrodes are arranged on the circumference of a surface of an living body to surround multiple muscle fibers; a first voltage Vi generated when a first external resistor is connected to between each electrode and a ground potential and a second voltage V′i generated when a second external resistor is connected to between each electrode and the ground potential; and a ratio Vi/V′i is calculated from the first voltage Vi and the second voltage V′i, and the position of a signal source in the living body is detected based on three ratios Vi/V′i.