Biomass resources are one of the attractive alternative fuels. It is effective to use such fuel in distributed power generation system with gas engine because it disperses. However, its calorific value is low, and the gas composition is affected by the source material, gasification method and gasifying conditions, causing it to change momentarily during engine operation. The gas engine must be stable and capable of high-thermal-efficiency operation under such fluctuations. The objective of this study is to develop a small gas engine system for biomass gaseous fuel. In this study, the effect of fuel composition on combustion was clarified by an engine experiment with low-calorific-value gaseous fuel. Diluted fuel exhibited longer combustion duration and lower thermal efficiency, but not necessarily a lower calorific value. It was also confirmed that spark timing control against fuel composition fluctuation was effective for maintaining high thermal efficiency even at a fixed speed engine. Fuels containing H2 also have lower calorific values, but they allow stable engine operation and higher thermal efficiency with respect to a shorter combustion duration.