PURPOSE: To facilitate control during switching of a twin turbo mode and to perform effective reduction of the occurrence of a shock when a sequential turbo type supercharger system is adapted to a horizontal opposite type engine.

CONSTITUTION: Primary and secondary turbo superchargers 30 and 40 are arranged in the vicinity of the right and left banks of a horizontal opposite engine body 1 and waste gate valves 31 and 41 are arranged on the two turbo superchargers 30 and 40, respectively. An exhaust gas control valve 45 and an intake air control valve 46 are arranged to the secondary turbo supercharger 40. Further, A precontrol valve 52 is arranged in a bypassing state to the exhaust gas control valve 45 of the secondary turbo supercharger 40, and the bores of the valve ports of the waste gate valves 31 and 41 and the precontrol valve 52 are set to the same value as each other.