To provide a structure easy in belt tension adjusting work in on-vehicle maintenance by improving mounting performance to a vehicle in a belt tension adjusting device of an internal combustion engine in this invention.

This invention is the belt tension adjusting device of the internal combustion engine, and is characterized by adjusting tension of a belt by relatively moving an adjusting bar to a bolt support bracket when rotating the adjusting bolt by forming a bolt fitting part having a screw hole fitted to a screw part of the adjusting bolt in the adjusting bar by holding the adjusting bolt by the bolt support bracket fixed to the internal combustion engine so that the axis becomes parallel to the extension direction of a fastening hole of the adjusting bar.