In a V-type six-cylinder engine, turbo-superchargers are provided for compressing intake air and feeding the compressed air into combustion chambers, and an ECU is operable to switch the combustion mode from a non-supercharged stoichiometric combustion mode to a supercharged lean combustion mode, depending on the engine operating conditions. When switching from the non-supercharged stoichiometric combustion mode to the supercharged lean combustion mode, the ECU retards the ignition timing, and keeps the retard amount of the ignition timing at a constant value if the increasing actual boost pressure becomes equal to or higher than a pre-set target boost pressure.