To provide a driving force transmitting mechanism which can surely transmit the driving force and has no possibility of obstructing smooth transfer for an object.

A driving force transmitting mechanism comprises a sprocket 23 rotated by the rotary drive of a driving motor 6, and a timing belt which is interposed between the sprocket 23 and a transport timing belt 21 and wrapped like a ring round the peripheries of a pair of belt rollers 25, 25 disposed at a designated space to engage the sprocket 23 with one side surface of a linear part between both belt rollers 25, 25 by applying pressing force thereto from the outside, so that the timing belt circulates and rotated round the peripheries of both belt rollers 25, 25. In the timing belt 20, the other side of the linear part between both belt rollers 25, 25 is engaged with the timing belt 21.