In a driving force transmission mechanism using a timing belt for an internal combustion engine according to the present invention, a face width of teeth of the timing belt at dedendums of the teeth is set larger than a width of engagement of a tooth of the timing belt with a tooth of a timing pulley. In the mechanism, a certain degree of freedom in accordance with elastic deformation is given to the width end portions of teeth of the timing belt, and thereby concentration of a load can be decreased on a corner of the dedendum of the teeth. As a result, breakage of the fibers of a canvas and cracking of the teeth of the timing belt etc. can be prevented, and the durability and the span of life of the timing belt can be increased.