This paper presents the performance and combustion characteristics of a spark-ignition gas engine power generation system fueled with a real biogas produced through a low-temperature biomass gasification process. The biomass resource was wood chips, which produce real biogas containing hydrogen, methane and carbon monoxide as fuels together with carbon dioxide and nitrogen as diluents. Real biogas operation gives much lower cyclic variability than that with natural gas. The maximum brake thermal efficiency was more than 30 %, even at the Brake Mean Effective Pressure less than 0.4 MPa. Thus, it is shown that the biogas is a suitable fuel for lean burn with dilution, which verifies the results obtained previously even quantitatively.