An exhaust system structure of an internal combustion engine, includes an exhaust manifold which is configured to be connected to a cylinder of the internal combustion engine, an exhaust pipe which isprovided on a downstream side of an exhaust flow from the exhaust manifold, a connection member including a flange portion which connects the exhaust pipe and the exhaust manifold to each other and aboss portion which is provided integrally with the flange portion and is fixed to a portion of an outer circumferential surface of the exhaust pipe, a sensor which is attached to the boss portion andis configured to detect components of the exhaust, and a purification device which is connected to a downstream end of the exhaust pipe and is disposed on a side opposite to the boss portion with respect to the exhaust pipe.