Event-related potentials (ERPs)—in particular, the P300 component—have come under increasing scrutiny as a means of clarifying brain functions, as they are considered to reflect higher brain activity. Although much research has been devoted to the P300 signal, its source within the human brain remains unclear. Electroencephalogram (EEG) is often used in ERP studies because it directly reflects neurological responses to external stimulus. However, owing to the low spatial resolution of EEG, many surface or intracranial electrodes are required to estimate the signal source. We have attempted to develop a new signal source estimation method using only one signal electrode, which utilizes the switching voltage divider technology for simultaneous measurement of the potential and location of the signal source;i. e., whether the signal source is far from or near the signal electrode. This signal source estimation method may improve the low spatial resolution of EEG. Using our proposed method, we measured the ERPs of subjects participating in an oddball paradigm using auditory stimuli. The results suggest that the P300 component of an ERP is generated in a region deep within the brain.