The invention provides a chain transmission mechanism capable of alleviating impact and impact noise caused when a chain engages and contacts with a sprocket, of not increasing the impact and impact noise even if the chain transmission mechanism is driven in heavy load and in high speed, of reducing vibrations, breakage, wear and others of the chain and the sprocket and of improving its durability. Tooth bottoms of the sprocket are formed so as not contact any pin, bush or drive roller when the chain is wound around the sprocket as guide rollers rotatably supported coaxially with a pin of the chain rolls on a track along the guide rail provided on the side of teeth of the sprocket. The invention provides a guide rail around the sprocket to engage the chain and lighten its impact with the sprocket as the mechanism operates.