A heat insulating structure of an internal combustion engine (engine 1) includes a cylinder-head-side heat insulating cover (30) and a cylinder-block-side heat insulating cover (40). Each of the first side walls (32) of the cylinder-head-side heat insulating cover (30) is disposed outwardly of, and is spaced apart from, a corresponding one of the second side walls (43) of the cylinder-block-side heat insulating cover (40) in the width direction of the vehicle. The lower edge of each of the first side walls (32) is positioned below the upper edge of the corresponding one of the second side walls (43) to overlap with the corresponding one of the second side walls (43) when viewed from the side of the vehicle.