  Magnetic fields can play an important role in the energy balance and
formation of gas structures in galaxies. However, their dynamical effect on the
rotation curve of galaxies is immensely unexplored. We investigate the
dynamical effect of the known magnetic arms of $NGC\;6946$ on its circular gas
rotation traced in HI, considering two dark matter mass density models, ISO,
and the universal NFW profile. We used a three-dimensional model for the
magnetic field structure to fit the modeled rotation curve to the observed data
via an $\chi$-squared minimization method. The shape of the HI gas rotation
curve is reproduced better including the effect of the magnetic field,
especially in the outer part, where the dynamical effect of the magnetic field
could become important. The typical amplitude of the regular magnetic field
contribution in the rotation curve is about $ 6 - 14 \; km s^{-1}$ in the outer
gaseous disk of the galaxy $NGC\;6946$. The contribution ratio of the regular
magnetic field to the observed circular velocity and to dark matter increases
with the galactocentric radius. Its ratio to the observed rotational velocity
is about five percent and to dark matter is about 10 percent in the outer
regions of the galaxy $NGC\;6946$. Therefore, the large-scale magnetic fields
cannot be completely ignored in the large-scale dynamics of spiral galaxies,
especially in the outer parts of galaxies.
