To provide a rotational power transmitting apparatus, a power transmitting belt engaging method, and a power transmitting belt, capable of making it easy to engage the belt, simple in construction and inexpensive.

A belt 3 is engaged between a crankshaft pulley 1 and a pulley 5 while the belt 3 having a predetermined slackening length at a normal temperature. Thus the belt 3 has a predetermined slackening length, which makes it easy to engage the belt 3. The belt 3 is heated as a whole by a dryer or the like, thereby causing the belt 3 to be contracted to remove slackening length, so as to generate tension of the belt 3. The heating sets the tension of the belt 3 within a suitable tension range, which makes the belt engaging work easy.