To increase rigidity of a support structure of an auxiliary machine of an internal combustion engine, for suppressing vibration transmission from the auxiliary machine to an internal combustion engine body.

A support structure of an ACG 60 of an internal combustion engine 1 provided with a cylinder block 2, a cylinder head 3, and a chain cover 6 includes: a bracket 75 provided at the chain cover and supporting a lower part of an auxiliary machine; an intake manifold fastening part 21 which protrudes at a rear sidewall 16 of the cylinder head and to which an intake manifold 24 is fastened; a cover fastening part 32 which is extended along a side edge of a right sidewall 12 of the cylinder head and to which a side edge of the chain cover is fastened; an upper ACG support part 51 protruding outward from the cover fastening part in the cylinder head relative to the rear sidewall and supporting an upper part of the ACG; and a first reinforcement rib 55 extended at the rear sidewall of the cylinder head and connecting the intake manifold fastening part and the upper ACG support part.