Methylcyclopentadienyl manganese tricarbonyl (MMT) is used as an octane-enhancing metallic additive for unleaded gasoline which can prevent engine knock by proactive reaction with the hydrocarbon free radicals before starting auto-ignition of hydrocarbons. But adversely it is pointed out that MMT causes catalytic converters clogged. Therefore, a lot of countries have the fuel standard to prohibit or limit the usage of MMT. However, still some countries use MMT as there is no fuel standard to restrict. As mentioned in several papers, metallic additives of engine oil such as calcium cause to make an abnormal combustion phenomenon which is called low-speed pre-ignition (LSPI) in boosted Spark Ignition engines. On the other hand, the effect of metallic additives of gasoline such as MMT on LSPI has not been studied. In this study, it was confirmed that manganese in gasoline increased the possibility of LSPI and the lower volatile gasoline which contains heavy aromatic components tended to show the higher sensitivity to manganese concentration in boosted Spark Ignition engines. And it was analyzed the amounts of manganese containing deposits increased in the combustion chamber after this evaluation. In addition, it was observed several particles became glowing in the combustion chamber and became the starting point of LSPI. Regarding to the mechanism of the phenomenon, it is supposed that the heat of reaction produced by the oxidizing MMT in gasoline droplets or manganese containing deposits is the ignition source and increases the possibility of LSPI.