Sound reducing device for internal combustion engine

A sound reducing device for an internal combustion engine that includes a crankshaft rotatably supported by an engine main body, a camshaft rotatably supported by the engine main body, a timing belt passed around a crank pulley fitted to the crankshaft and a cam pulley fitted to the camshaft, and a belt cover attached to the engine main body to define a belt chamber for accommodating the timing belt therein jointly with the engine main body, the sound reducing device comprising a chamber forming member defining a sound reducing chamber having a prescribed column length jointly with the belt cover, wherein the sound reducing chamber communicates with the belt chamber via an opening, and the opening opposes a part of the timing belt passed around the cam pulley.