To provide a magnetic cable elongation sensor which can be installed even if the interval between underground cable line openings in a manhole is narrow.

This magnetic cable elongation sensor 10, by which the amount of elongation of a cable 3 passed through a line is calculated by means of a magnetic force and output as an electric signal, includes a magnet 7 provided in a predetermined position on the cable 3, a magnetic sensor 11 provided to face the cable 3 and comprising a number of elements 12 aligned, and a processing part 13 which, from changes in output of each element 12 of the magnetic sensor 11, calculates the position of the magnet 7 for output as an electric signal. This constitution allows the magnet 7 to move as the cable 3 moves axially, resulting in a rise in output of the element near the magnet 7. The rise in output to the processing part 13, which in turn calculates the position of the magnet 7. The amount of elongation of the cable 3 is calculated from the position of the magnet 7 after the movement.