To provide an elongation detection device for a tread chain of a passenger conveyer capable of detecting abnormality of the tread chain in an early stage and specifying the chain abnormal part to enable a simple maintenance operation.

This device calculates change of output value from a tread passage detector 1 with an operation piece 2 provided on a tread, the tread passage detector 1 provided on a fixed part in a passenger conveyer, and a device 7 detecting speed of the tread or a moving handrail, and judges that the change of output value exceeds a designated value to detect an abnormality of a tread chain in an early stage. If an abnormality of the tread chain occurs, the abnormality of the tread chain is detected in an early stage with a small measurement error and an abnormal part is specified by the tread passage detector 1 to enable the early measures and to maintain a quality of a passenger conveyer.