Methanosarcina alcaliphilum which is alkalophilic methanogen having the optimum growth pH range of from about 8.1 to about 8.7. This strain exhibits excellent properties such as alkalophilicity and resistance to low temperature. This strain makes it possible to carry out methane fermentation methods at an alkaline pH and at a low temperature conditions. This strain is applied to methane fermentation in treating solid waste and waste water. Moreover, the bacterial concentration thereof in a reactor can be increased within a very short time and, therefore, the size of such a reactor can be made compact.