Methane stored in a natural lake sediment was investigated. Fresh sediment (8--25 cm in sediment depth) taken from Lake Biwa, Japan, contains 2.6 ¿mol/g d.s. (dry sediment) of CH4 on average. The sediment retained 0.31 ¿mol/g d.s. of CH4 even after air drying, which is 12% of the total CH4 in the fresh sediment. The higher the temperature for drying of the sediment, the smaller the amount of CH4 retained. The result of an adsorption experiment indicated that dried and completely degassed lake sediment adsorbs CH4. Clay particles in the sediment are possibly responsible for the adsorption of CH4. Methane observed in the fresh lake sediment was probably produced by bacteria in the past, then adsorbed on the sediment particles. Adsorption is another form of storage of CH4 in lake and ocean sediments, in addition to the well known forms of dissolved, bubble, and hydrate CH4.