To improve ignitability of fuel of low compression ignitability in a combustion chamber during compression self ignition diffuse combustion mode operation.

A multi-fuel internal combustion engine operated by introducing at least one kind of fuel out of at least two kinds of fuels F1, F2 having different properties to a combustion chamber CC or introducing mixed fuel of at least the two kinds of fuels F1, F2 to the combustion chamber CC, is provided with a fuel property detection means (electronic control device 1) detecting ignitability index value which indicates compression ignitability of fuel itself introduced to the combustion chamber CC as an index, and a fuel injection control means (electronic control device 1) introducing fuel of low compression ignitability into the combustion chamber CC by injecting fuel before a predetermined time in a period from a suction stroke to a compression stroke and executing main injection after that when compression self ignition diffuse combustion is executed by using fuel in the combustion chamber CC which is judged as fuel of low compression ignitability based on the ignitability index value.

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