To measure and inspect the stranding period of a cable in the stranding process without measuring with a sample.

A signal is given only to a specific objective insulating electric wire of a plurality of insulating electric wires of a stranded cable 10, while the cable 10 during stranding is moved in the axial direction, the variation of electromagnetic or electrostatic effect on the environment given by the signal of the objective insulating electric wire is detected with a sensor 11 approached to the outer surface of the stranded cable 10. The variation of the separated distance between the sensor 11 and the objective insulating electric wire stranded in the stranded cable 10 is detected, and the stranding period of the objective insulating electric wire is computed and detected based on the detected variation of separated distance and the moved length in the axial direction of the stranded cable 10.