This paper describes about the method for detecting chain elongation.,the chain system is widely used for power transmission device. Commonly,It is known that if the chain enlongates ,the position of the chain-rollers in the sprocket tooth move to tooth tip. And the most papers show that if the chain have a uniformity enlongation over the whole length, all chain-roller in the sprocket tooth move the same amount. On this time , we look into the structural feature of the chain , and we verify the videograph of the camera fixed at position of specific tooth on the sprocket, so we can elucidate the next two characteristics. (1)if the chain enlongetes, the link enlongates on every other link. (2) if the chain enlongetes,on the sproket the tooth that chain-roller is on the pitch circle exists certainly. With these as assumption, we verify the engagement condition of chain-roller and sprocket tooth in the geometrical techniques with CAD. As a result, we confirm that if the chain enlongetes, the different of engagement hight between the neighboring chain-rollers and this different correlates with the elongation amount. We develope the enlongation amount measuring method, and verified the effect of this method with the experiment.