The objective of measurement of bio-signals in measurement uncertainty is not to determine the true value as closely as possible, but to determine a measured value and to assign the interval of the value. The measurement uncertainty is estimated by type A and B evaluations, depending on whether they are evaluated by statistics or the mathematical probability theory. Intraoperative neurophysiologic monitoring is used often for early detection of inherent risk relevant to neurosurgical procedures leading to permanent neurological injury, while it is still potentially reversible. In this study, we evaluated the uncertainties in somatosensory evoked potentials (SSEPs), which are used for monitoring sensory neural pathways. In a 45-year-old man who underwent cervical laminectomy, SSEPs were monitored using the ISIS IOM SYSTEM (Inomed, Emendingen, Germany) to evaluate the uncertainties. Expanded uncertainty were 0.88 mV and 1.22 ms, for amplitude and latency, respectively. Measured values and corresponding uncertainties of amplitude and latency were 2.78 ± 0.88 mV and 24.02 ± 1.22 ms, respectively. The expanded uncertainty (0.88 mV) of the amplitude was approximately 30% of the mean value (2.78 mV). A reasonable explanation for this would be the effects of variables such as electromagnetic waves (diathermy and warming blankets), temperature, blood pressure, sex and body mass index on SSEPs. Careful attention is required in interpreting SSEPs.