To provide a measuring method and device for axial elongation capable of precisely measuring the axial elongation of a rotating shaft regardless of the magnitude of elongation.

The rotating shaft 1 to measure the axial elongation has grooves 10 and 12 inclined in mutually opposite directions to the axial center on the rotating surface. A sensor 14 is arranged opposite to the rotating surface of the rotating shaft 1. According to the rotation of the rotating shaft 1, the sensor 14 generates pulses every passing of the grooves 10 and 12. Since the circumferential space between the grooves 10 and I2 is varied depending on the axial position of the rotating shaft 1, the pulse interval generated by the sensor 14 is changed when the position of the grooves 10 and 12 in the position of the sensor 14 is changed by the axial elongation. The axial elongation is determined from the change of the pulse generating interval.