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 the auto-ignition of hydrocarbons. However it has been pointed out that MMT causes automotive catalysts clogging and spark plug severely fouling. Therefore, many countries have fuel standards that prohibit or limit the usage of MMT. Nevertheless, some countries still use MMT as there are no restrictions imposed by fuel standards. As mentioned in several papers, metallic additives of engine oil such as calcium cause an abnormal combustion phenomenon called low-speed pre-ignition (LSPI) in turbocharged spark ignition engines. In contrast, 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 that lower volatile gasoline which contains heavy aromatic components tended to show higher sensitivity to manganese concentration in turbocharged spark ignition engines. These analyses also showed that the amounts of deposits containing manganese increased in the combustion chamber. In addition, it was observed that several particles became glowing in the combustion chamber and became the starting point of LSPI. With regard to the mechanism underlying the phenomenon, it is supposed that the heat of reaction produced by the oxidizing MMT in gasoline droplets or deposits containing manganese form the ignition source and increase the possibility of LSPI.