To provide a control unit of an internal combustion engine capable of leaning ignition timing for promptly avoiding knocking even when fuel properties are varied.

The internal combustion engine comprises a combustion chamber 18 in which a mixture of at least two sorts of fuel of different properties and air can be burned, an ignition plug 45 for igniting the mixture, a knock detection means 63 for detecting knocking and an air-fuel ratio detection means 61 for detecting an air-fuel ratio and is characterized in that the ECU 51 is provided with an ignition timing leaning means for updating limit ignition timing causing knocking, which varies according to the air-fuel ratio, based on a learning value to a retard side when knocking is detected and with an ignition timing control means for controlling ignition timing by the ignition plug 45 based on the limit ignition timing causing knocking updated by the ignition timing leaning means.

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