Natural methane hydrate under deep ocean floor in Ocean Drilling Program (ODP) Leg 164 was studied by electron spin resonance (ESR) after gamma-ray irradiation. The methyl radical (g=2.0024+/-0.0005, A=2.3+/-0.1 mT) as well as an unidentified signal was observed. Although the parameters and the thermal stability were very similar to those in a synthetic sample, the signal intensity was about one eightieth of that in synthetic one because methane hydrate had partially changed to ice. ESR signal intensity of the methyl radical after gamma-ray irradiation could be used to estimate the amount of methane hydrate in a sample mixed with ice.