To reduce the working cost of a formed tube by recognizing the degree of damage of an insulating resin around a lead wire quantitatively, diagnosing service life of an electromagnetic forming coil with high precision on the basis of the degree of and decreasing the wasteful manufacture of the electromagnetic coil which is used for working the formed tube.

The electromagnetic forming coil 1 is provided with an electromagnetic coil 11 abutting on the circumference of a core material 10. In the electromagnetic coil 11, the conductor wire 11a is wound around into a coil shape and the space between adjacent conductor wires 11a is coated with a resin 11b and insulated. On the electromagnetic coil 1, a sensor 12 is provided in the boundary region between the core material 10 and the insulating resin 11b. Shear strain which is caused between the core material 10 and the insulating resin 11b is measured with this sensor 12 and the service life of the electromagnetic forming coil 1 is predicted on the basis of the measured value of the shear strain.