To provide an auxiliary chamber type internal combustion engine capable of preventing unburned mixture in a main combustion chamber from reverse-flowing into an auxiliary chamber or an efficiency from lowering and exhaust gas properties from deteriorating due to the backflow of the unburned mixture in the main combustion chamber into the auxiliary chamber.

This engine comprises the main combustion chamber 21 and the auxiliary chamber 34. The auxiliary chamber 34 is allowed to communicate with the inside of the main combustion chamber 21 through a flame jet flow passage 36 and an injection port 14 for flame jet. The engine also comprises a backflow prevention means (check valve 15) preventing the unburned mixture in the main combustion chamber 21 from reverse-flowing into the auxiliary chamber 34 and a backflow mixture combustion means (control device 30 and a control unit 30a) re-igniting reverse-flown mixture.

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