Biogas is mainly composed of methane (CH4) and carbon dioxide (CO2) as presented in Chap. 1. Spark ignition reciprocating engines are often used for biogas engine because of only small modifications of fuel supply system from gasoline engine especially at small output powers. At first, spark discharge, burning velocity and flame structure are explained. Next, the effects of CO2 ratio, equivalence ratio, compression ratio, hydrogen addition, exhaust gas recirculation (EGR), fuel property and other physical parameters are shown based on the literature sources published. An example of analyzing method of combustion by using pressure history data is introduced combined with NOx (oxides of nitrogen) emission data. When CO2 ratio in the fuel increases, the burning duration becomes longer, and the burning rate becomes smaller due to smaller laminar burning velocity. Abnormal combustion of knock must be avoided in spark ignition engines. Autoignition and knock behavior in the end-gas region is visualized with a high-speed camera. Methane number as an indicator of resistance to knock is discussed. Pre-chamber with small holes, from where burned gas jets come out and make several ignition locations in a main chamber, is used in lean burn, large-sized bore engines because of shorter flame development time.