  We studied large-scale magnetic field reversals of a galaxy based on a
magnetic vector map of NGC6946. The magnetic vector map was constructed based
on the polarization maps in the C and X bands after the determination of the
geometrical orientation of a disk with the use of an infrared image and the
velocity field, according to the trailing spiral arm assumption. We examined
the azimuthal variation of the magnetic vector and found that the magnetic
pitch angle changes continually as a function of the azimuthal angle in the
inter-arm region. However, the direction of the magnetic field had $180^\circ$
jumps at the azimuthal angles of $20^\circ, 110^\circ, 140^\circ, 220^\circ,
280^\circ$, and $330^\circ$. These reversals seem to be related to the spiral
arms since the locations of the jumps are coincident with those of the spiral
arms. These six reversals of the magnetic field were seen only in the inner
region of NGC6946 whereas four reversals can be identified in the outer region.
