A torque coil (two-layer coil structure) of the disclosed embodiments includes an inner coil formed by spirally winding metal wire, and an outer coil arranged to be in close contact with the outer periphery of the inner coil and formed by spirally winding metal wire. A winding direction of the inner coil and a winding direction of the outer coil are opposite from each other, and when the torque coil is twisted in the circumferential direction in which the diameter of the inner coil is increased, a change amount of the diameter of the outer coil is larger than a change amount of the diameter of the inner coil. The two-layer coil structure has high twisting rigidity, thus suppressing the occurrence of kinks due to rotational resistance.

The invention provides a steel cord with a construction of n1, n is the number of the steel filaments of the steel cord, the steel cord has an elongation at 2.5N-50N of less than 1.2% and a twist pitch of greater than 16 mm, each of the steel filaments has a form of helical wave with a wave length L expressed in mm and a wave height H expressed in mm when being unravelled from said steel cord, L is greater than 16 mm, each of the steel filaments has a space volume Vs satisfying that, Vs=LH.sup.2/4, and Vs>20 mm.sup.3. The invention steel cord is beneficial for the stress distribution.