This study is aiming at an optimal use of bio-gas for the small distributed power-generation system making good use of its characteristics. The diesel dual fuel (DDF) operation is paid attention, which is easily remodeled from a diesel generator on the market to supply low-calorific gaseous fuels. Though there are many studies focused on the performance of DDF operation when bio-gas is supplied to diesel engine, the response to momentary change of demand of electricity or amount of gas supply is not guaranteed. In this study, according to the background above, the performance or stability of output on regular DDF operation with methane is, first, examined by experiment and specific characteristics of the diesel generator is, secondly, examined on the transient operation like momentary change of load or methane supply. As a result of experiment, it is found that the diesel generator which is used through this study is highly able to follow the momentary change of load or gas supply, and the combustion of methane progresses steadily at the both conditions of small quantity of oil (large quantity of methane) and the large quantity of oil (small quantity of methane).