A non-invasive ESR measuring method for in vivo kinetic studies of a nitroxide radical in the liver of a rat by combining a surface-coil-type resonator (SCR) and field gradient method was developed. The depth across which a microwave magnetic field (B-1) can penetrate under the SCR was about 4 mm. Using the SCR, ESR measurements without any invasive surgical procedures were made on the upper abdomen of a rat that received an intravenous injection of a nitroxide radical under a field gradient along the B-1-direction. It was found that the decay rate of nitroxide radical 4 mm under the skin (i.e., in the liver) is fast compared to that near the skin.