An internal combustion engine comprising a main intake passage supplying air-fuel mixture to a combustion chamber, an auxiliary intake passage supplying air or an air-fuel mixture to the combustion chamber through an injection port, a fuel supplement passage for supplementally supplying fuel to the auxiliary intake passage, a solenoid valve provided in the fuel supplement passage, and a control device for controlling opening and closing of the solenoid valve in response to a signal from an oxygen sensor disposed in an exhaust passage, said solenoid valve controlling supply of fuel to the auxiliary intake passage, thereby regulating an overall air-fuel ratio of the mixture supplied to the combustion chamber approximately to a stoichiometric air-fuel ratio.