To provide a tension adjusting device capable of restraining rapid increase of tension on a tension side of a timing chain caused by rapid rotation fluctuation of a crank sprocket with a simple and inexpensive structure, capable of surely preventing bad influence on the timing chain, a crank shaft and a cam shaft, and capable of improving versatility.

A guide member 33 and a lever member 34 are arranged along a bridged part 23b on a rotation direction downstream side of a lower V shaped zone of the timing chain 23 wound around the crank sprocket and a cam sprocket. The bridged part is guided from the outer side by a crank sprocket side end of the guide member rotatably supporting a cam sprocket side end and working as a free end side. The cam sprocket side end of the lever member to which the crank sprocket side end is fixed is superimposed slidably over a timing chain rotation direction predetermined region R with respect to the crank sprocket side end of the guide member from the outer side, and bending force is applied.